There have been plenty of calls in recent weeks to revisit the 1922 Committee rule whereby PMs cannot be challenged again for a year, with some Tories (e.g. Geoffrey Clifton-Brown, the committee's treasurer) openly saying they'd vote against May. And as noted in the comments, a few Conservatives have defected to ChangeUK in the past few months. Also, she had an unruly meeting with her cabinet in recent days, and Andrea Leadsom ultimately resigned from her government, basically refusing to present May's "new" Brexit bill to Parliament.
The last time there had been a leadership challenge within the Conservative Party, May's cabinet was still behind her, so it was basically the ERG voting against her and the others voting to support her. If the cabinet infighting reported in the news is anything to go by, that is no longer the case.
The last time there had been a vote of no confidence proper in Parliament, Tories and the DUP sided against the opposition to avoid a new election. It's anyone's guess how that would have worked out had it occurred -- on the one hand side it's hard to imagine Conservatives wanting to trigger a new election, but on the other the Tories really didn't want May anymore.