The short answer is that it's not sudden. Over the past few years China has been increasing its influence locally and trying to have more influence globally in a similar way to the US. This increasingly brings China and the US in to direct competition in a number of areas. As China's power and influence increase, more and more politicians in the US will gradually change from viewing China as a manageable, economic challenge to the US and become more of a genuine geopolitical rival that (from the US point of view) needs to be checked.
In more detail, in recent years China has taken a number of initiatives to assert increasing control of the South China Sea. It has always claimed the deliberately vague nine dash line:
But in recent years it has taken more steps to enforce that claim, which is vital to China's maritime trade routes. It has embarked on a process of rapid modernisation and expansion of its navy
The People’s Liberation Army Navy (PLAN) capabilities will surpass most other navies by 2020. China would have as many aircraft carriers as Britain and India, more nuclear attack submarines than either Britain or France, and as many AEGIS-like destroyers as all the other non-US navies combined. China would have two aircraft carriers, 20-22 AEGIS like destroyers and 6-7 nuclear attack submarines, while United States would have eleven aircraft carriers; 88 AEGIS like destroyers; and 48 nuclear attack submarines. While China would still be considerably behind the US Navy, its growing capabilities could already begin to have significant consequences for the United States and other countries.
(taken from here)
It has also been building and expanding artificial islands in e.g. the Spratly Islands to support airstrips, naval facilities and anti-ship missiles to control access in the region
China appears to have deployed long-range anti-ship cruise missiles and air-defense missiles to the Spratly Islands, providing the islands with offensive reach for the first time. The move dashes hopes across the region that China would refrain from, or at least limit, its militarization of the extensive bases it has built on reclaimed coral reefs in the heavily disputed islands.
(taken from here)
In a similar vein, China has been in a high profile dispute with the Philippines over oil exploitation in Philippines' territorial waters, with China essentially asserting that the Philippines can only develop there with China's permission and involvement. President Duterte of the Philippines is notably closer to China than previous Philippines' presidents who have been very tightly aligned towards the US. The two countries have recently come to an agreement on the subject, but the view seems to be that China's negotiation was backed up by the none-too-subtle suggestion they'd be prepared to use the military to assert their claims if necessary.
Further afield, China has been increasing diplomatic, economic and military influence elsewhere. It recently promised to defend the Maldives from Indian interference in the Indian ocean, has been tension with India in 2017 over Sikkim in the Himalayas (in terrain not well suited to any real fighting) - which has lead to some tense diplomacy between the countries.
There has also been the One Road One Belt initiative which is an attempt to create a trade network centered on China that spreads through Eurasia.
On 4th May 2018, the US accused China of using lasers to deliberately injure USAF pilots in Djibouti. Whether that is true or not I don't know, but it does serve to emphasise that China has military bases around the world, such as in Djibouti, to protect Chinese interests.
Politically, the recent Chinese communist party conference in effect gave President Xi Jinping more power than any individual has had in China since Mao's death. The Party in fact adopted a deliberate policy of preventing individuals from gaining that much power as a reaction to what was seen as Mao's catastrophic use of it. That President Xi has accumulated so much personal power is partly a reflection of his political acumen, and partly a recognition by the Party that China needs a single strong leader at the moment as it transitions from a weak local power recovering from the disasters of Japanese occupation in the early 20th century followed the cultural revolution and the great leap forwards, and instead becomes a global power.
Put it all together, and you have a country that has put itself on a path to becoming a global power, and is consciously taking steps to secure and advance its position.
Since the US is currently in the position of being the sole global superpower, it is inevitably going to oppose anything that even slightly threatens that. Currently all shipping and naval activity in the world takes place with the implicit consent of the US navy (in that it is capable of dominating any areas of the ocean it wishes to). For the US, China having the ability to take that role away in the South China Sea is an unacceptable threat to US interests.
This is something that has really only started to develop in the last few years. And as China's power grows, US politicians, one by one over time become aware and increasingly concerned, and start to raise those concerns publicly.