I'm also confused by Jess's assertion months back that Rob had slept with her on Valentine's day (2015). If he had raped her as well, why would she have told Helen in such an ownershippy kind of way?
It isn't always that obvious.

Scenario:
Knob goes to see Jess. Woos her with champagne. Tells her he wants her back.
Makes a move on her. She's not wild keen. But he 'doesn't take no for an answer'. So she goes along with it.
In the morning he says it was just sex.
He's with Helen now.
And Helen is better in bed.
As far as we know, she didn't even know about Hellin up until this point.
Jess is so used to being sex being 'taken' from her rather than volunteered that she doesn't even see it for what it was until much later.
Pride hurt by finding about the OW she sleeps with an old flame.
Then she decides fuck it two can play at this game and turns up at BHC on Velnetine's night to woo Rob. Remember she was surprised H had moved in and that Rob was out. The bed was barely cold from her moving out.
Then she realises that not only has Rob moved in with OW he isn't even with her on VD so clearly there is another other woman.
She tells H about last week as an act of revenge on the woman who has stolen her dh and who is also allegedly better in bed but who is also obviously being duped by Rob.
The only bit that doesn't make sense is if she did know about H before VD but didn't know she was at BHC then presumably she'd have expected them both to be out or both on together