Was he cheating?

Hii i’m pretty devastated at the moment because i just found an Hickey on my boyfriends neck i asked him what it is and where it’s from and he started laughing saying i’m stupid because it is nothing , he said it’s not an Hickey but he has also no idea how it got there and where he got it from . He also tried too calm me down and gave me a lot of kisses but i always pushed him away . I don’t know what too believe, i kinda believe my boyfriend because he promised he would never cheat on me because he loves me so much , but on the other hand if he has no clue how that mark got there it can only be a hickey . I mean if he would’ve hurt himself there he should remember that right? I’m also really devastated because tomorrow is my Birthday and if he really just cheated on me one day before my birthday that’s disgusting...i’m so sad and mad at the same time..i have no idea what too do or what too believe