Why would he lie to me about hanging out with his friend??

I been dating this guy for about 1.5years a lot of ups and downs though. He has a girl best friend and I’m like ok cool…

last year he went to a museum but In his pic I saw a girl foot so close to him so I’m like who’s that she not with you and he said no..

I find out recently that was a lie he was with his friend but why would he lie about hanging out with her if he would’ve just said that It would’ve been ok with me. I never accused him of lying, always gave him benefit of the doubt but now I actually have proof that he lied in fact he’s been going out with her all year round! Every holiday he disappear, didn’t even tell me happy Mother’s Day knowing I’m a mother.. I feel like ever since I got pregnant exactly a year ago by him everything just been going downhill we haven’t even gone on dates recently either he always say he has work but he still comes over to have sex.. and he also makes up excuses now for me not to go over by him which is also weird!! I used to always go over by him…

I want to know if they’re doing it 😫 like ok he lied.. what else have he been lying about then I feel like they’re at a point of intimacy or am I just assuming cause they’re close enough. Cause why they been going out and he hasn’t been taking me ugh.

Also all she does is posts about her and her friend , showing all the dates they been going on etc it actually seems like she probably live right In the same house as him but in another room based on how the pics look it look just like his room just another room or something

Update: yeah turns out they were fucking, she’s actually pregnant right now due end of Nov. me and him don’t talk anymore and I’m pregnant myself 🙃 how crazy has this year been for me..