(30f)Girlfriend is being shady.

Hey, so ive been dating this girl for about 4 months things are good. shes a very attractive women so guys are always trying to get with her it doesnt bother me at all because i trust her, she also has a couple of guys in her dm (IG) that she talks to i dont really say much but i do keep that in mind. 2 days ago she was off work and she barely has off work so she went to the beach... i asked her who she went with she said by herself. My girlfriend loves the beach so i was like okay cool. Yesterday she was also off from work and shes at the beach again.. i asked her who she went with and she said by herself. Now im just thinking okay thats weird why are you going to the beach yourself?? shes facetimes me while at the beach and invites me. i get back to her on it in about 15 mins and she told me to not come because it was going to take me a hour to get there and she didnt want to stay for that long... she ended up staying 3 hours after that conversation probably more this was just off what she posted off IG. i decide to long into my other IG to check the guy that dms her story and he just happens to be at the beach as well. she then disappears for about 5 hours and writes to me later in the day telling me her phone died. she calls me various time and (i was prepared to break up with her because it has become a thing that on her days off work she rather be her friends or someone else disappear on me and its like we dont even do anything anymore just work and sleep together and back to work again it just starting to turn me off and make me feel like shes up to something). i met up with her she was really happy to see me and was all up on me asked me if i missed her and why does it feel like that i dont. i was just off cause i dont know what to believe. i was going to break up with her but her girlfriend was there and i just feel like its something we need to talk about in private.

i do plan on breaking up with her later today or is there a different way i should approach this? am i overreacting?