I Think My GF Might Be Fucking My Dad

So yeah.... as the title says - I think my GF might be fucking my dad. I haven't caught them in the act, and I Haven't seen any texts, but I have picked up on little things. A few times I have been in bed while my girl has been taking a shower, and gone to the bathroom as soon as she came back - only to find that my dad is in there. Despite me not hearing him enter it.

For my latest birthday (20 y.o) my dad gave me a phone - he gave my girlfriend a car when she turned 20, saying that "turning 20 is a big deal". And I have noticed that my mom doesn't seem to like her at all any more. When I met her when we were both 17 she absolutely loved her, but that seems to have changed.

And the last thing I've noticed is that they're a little bit too friendly with each other. They're hugging, I've caught him looking at her tits and but, and she's started wearing clothes that are a bit revealing when at our place.

So yeah - I'm thinking that maybe they're fucking. Has anyone else ever encountered this? I'm not really sure how to feel about it, or if I'm even just being paranoid.