What are her given reasons? Could it be posturing for when her term is over?
Your original title question was a tad misleading/presumptive: she has never rejected all forms of healthcare reform. She has only rejected the most recent handful of attempts made by Republicans in the last several months.
She's had several interviews where she expounds on her reasons. This CNN article includes several quotes from her (emphasis and link mine):
"I'm concerned about the impact on cost and coverage," she said. "We already have a problem under the Affordable Care act with the cost of premiums and deductibles, and finally, I'm very concerned about the erosion of protections for people with pre-existing conditions."
Collins' concerns over Graham-Cassidy have ranged across a host of issues, including the way the bill would end Obamacare subsidies and Medicaid expansion, and instead give states a set amount of funding through block grants. Collins has said that she fears Graham-Cassidy might also take away protections for people with pre-existing conditions if insurers in certain states are allowed to charge more based on a person's health history.
"I'm reading the fine print on Graham-Cassidy," Collins said, according to the Press Herald. "The premiums would be so high they would be unaffordable."
These are essentially the same objections she has raised on all previous incarnations of repeal (and replace): she wanted to see the CBO score, and considered many millions of people being projected to lose coverage as a deal breaker for her. And every proposed bill has been projected by the CBO to do exactly that.
It's impossible to say if her publicly stated opinions and reservations are somehow different from her privately held ones, unless someone who has had private conversations with her about them suddenly produces recordings of them. And, arguably, as a politician her personal opinions should be largely irrelevant: she's there to serve the interests and wills of her constituents, not herself.
As irregardless' answer mentions, there's the possibility that she has aspirations for other political offices, though there's nothing official to corroborate this. She isn't up for re-election to the Senate until 2020, meaning she does not have to consider herself pressed to impress and appeal to constituents to retain her position right now. She can simply wait this out. She has neither need nor incentive to posture right now.
One of the reasons would be that there is speculation that she may run for governor of Maine, effectively appealing to her state's primary political views.
Ideologically, Susan Collins isn't actually that much of a Republican. If you look at the ideology score given by gov track, she's actually more liberal than a couple of Democratic Senators.
Otherwise, unless you know Susan Collins personally, we will never know for sure why she voted against repealing and replacing Obamacare.