Did my girlfriend cheat on me ...?

if you are that concerned then yes i would get a dna test. but sometimes certain traits such as skin pigmentation skip a generation or two. if your parents or grandparents were darker then it could have been ped down through them and been dormant in you. there have even been cases where two white people have a child and the baby looks black simply because one of the parents had an african american grandmother! genetics are extremely complicated and can surprise you at times. but just get the dna test done to be sure.