Things have changed -"She still doesn't like to kiss and there is absolutely no sex"

Just recently my girlfriend has started acting distant and strange.
She doesn't like me looking at her while were doing the sexy and she's all but stopped wanting it.

She won't say I love you unless I say it or she will ask me why most of the time. Now here's the strange bit.. We had an argument a few days ago about how things have changed and she said that she was upset I hadn't been cooking dinners and breakfast on Saturday mornings.

We made up after I promised to be more attentive and helpful which seemed great. She still doesn't like to kiss and there is absolutely no sex. Its as if she is decided we are friends sleeping in the same bed.

Let me go back 2 weeks.. We went to a dinner party with people we didn't know and the host was very outgoing and I kept catching him staring at her and they went outside to smoke and when I cane up they hushed talking.

I've been noticing that whenever she's on Facebook messenger during the day so is he! Am I just being paranoid or is this a building scenario?