Is my gf cheating?

my gf and i have been dating for several years now and dont have sex often. but the last 3 or 4 months we have had no se. my bday past by with nothing and it dosent seem like a priority to her. i see she buys more and more new clothes and i just worry maybe theres someone else? im around 4 inches and when we first started dating i found magnums in one of her drawers.