Stretch marks or dog scratches?

Justice
Hello my Is Justice I'm 19 years old. 
I've been with my boyfriend for a year and 3 months 
Yesterday I noticed something on his stomach, to me they looked like hickies
I asked him about it and he said that they were either stretch marks or dog scratches but I know what stretch marks look like. I have them on my legs. He hasn't been unfaithful but I have caught him talking to other girls behind my back , just basic flirting but no sex or anything like that. But I have this feeling he did something but is afraid to tell me and that he doesn't want to lose me.  Should I ask him about it again?