cant stop overthinking, please help me

Me F19 and my bf M20 have been together for a month now and everything was perfect. the reason why i said was is because he broke up with me. its a long story so ill make it short. ok so i forgot when but he lost his phone and hasnt been able to talk much like we used to. i think its been a week but im not sure. so he came back bc he was using his brothers old phone. (for context). now here comes the issue. he left me on delivered for 5 hours and hes very busy so i didnt think anything of it. and then i went on snap i noticed his snap score was going up by so much and it usually doesnt go up by that much. i brushed it off and went back on snap and saw a gorgeous woman on his story.... and so i went off on him without him giving me an explanation or anything and i asked his friend who knows him and she said that the girl was his sisters gf and i thought she was lying so i still accused him until he broke up with me. i knew he was going thru his own stuff and i pushed him and got him pissed. i never seen him so mad before. im still overthinking. why didnt he tell me beforehand? is he cheating or is it really his sisters gf?