My boyfriend has his cuzin as a screen saver but she have naked .that same night i had sex with did i mess up?

hi my name is silvia i am really trying to figure out if my guy is cheating on me or what i am15 and i just lost it last night i know im dumb but like i really love this guy and well last night i saw this picture of some girl on his phone and well i was like who is this and he told me its mu cuzin and i was like this girl is like half naked and well shes on all his screen savers i was upset and well i wanted to cry because i mean i don't really believe him so can some one please help before i keep on having sex with this guy and he do sent even love me..!!!