"i don't need to be assured. i need you to stop to talking about it"
for me she talk about indiscreet / uncomfortable questions, not sex
I think something happened between them the last time they shared a bed. We know Jill is a hugger, so it could be she dry humped Bella in her sleep. It could be something less sexual. Maybe Bella has night terrors and hit Jill.
The key lines are 'It's not going to happen again.' and 'It's not my fault, I was going to sleep on the couch.'
Whatever the issue is it happened before when they shared a bed.
I don't think at the moment that Jill would pry with Bella. She knows Bella's limits and advises MC not to ask the next day. She also isn't immediatly worried about Bella, it's not until MC points out the locked door that she gets concerned.
Is it 100% certain that it's sexual? No. But I think something more than an awkward conversation happened. Otherwise it would be something that came between them all the time and not just when they shared a bed.