Open Question: Why do I get blisters in my mouth after kissing?

so me and my girlsfriend just hit second base....and all of sudden I've been finding blisters in my mouth. the first one was by the bottoms of my gums by chim, then the second one was on my tongue. Those are the only two I've found and they went away pretty fast But I was wondering if these blisters could be a result of making out. I'm pretty sure she doesnt have an STD and theyre not like