What to do when you think your woman is cheating

I’ve been with this chick for a year now and our relationship has been sort of shaky. She was texting her ex in the beginning of the relationship (they only dated for 2 months and had been broken up for about 3 months before we started dating.) I confronted her about me being uncomfortable with her talking to him and she said she would stop. Then a month later I saw a text convo with the two of them flirting. Then I told her enough is enough and I would end it next time it happens. It happened again, and I just got upset and angry but never did anything. This was just one case of her being sketchy with other bros. So this week she is on vacation with her family and a bunch of family friends at a high end resort in the Caribbean. Shes barely talking to me for the last week which is fine considering we both needed some time apart and I have more time to chill. However the weirdest thing happened when I asked her what she did last night. She said “I went to bed at 1030 then woke up 1 and couldnt fall sleep till 3.” I was like ok, but what did you do not when did you go to bed. With that she tells me she was at dinner with her family and then went shopping and got my birthday gift.

I find it really strange that she was sleeping because her twin (fraternal) sister was snapchatting me at midnight and looked like she was in the lobby or something definitely not bed. My gf already knows that I have trust issues because of all the crap she did with her ex and all the other guys i never mentioned (3 or 4 different guys)

Anyway I’m asking my fellow brethren what they would do in my spot. Also I have no real proof shes doing anything other than how weird shes being while talking to me and lying about going to bed early for whatever reason i have no clue. Please share your wisdom.