Do you think my boyfriend is cheating on me?

So a couple days ago my lil sister txted my bf off her phone and was flirting with him and he flirted back(she made a fake name and acted like someone else) and then he told me that he was just tagging along to find out who it is he asked for pics of her and then he was saying shes hot and crap. then she would bring me up and hed say "lets not talk about my gf idc about her" and then last night he was texting his EX and this girl named Shay and he refused to let me see their txts to each other he said i need to trust him and i do i really do and then he got mad cuz i kept trying to read them and hed hide his phone then he got up and erased all his txts from them and i saw a picture of her and she was gorgeous he said shes pretty but shes fat and idk what to believe. I Love him but i dont want to get hurt.. but i was curious and now im kinda iffy with my trust and idk what to do! I need advice ASAP.. please help dont be rude...