Is this herpes or something else? Please help!

ive only got one blister(?) im not sure what it is, it looks more like a canker sore you would get in your mouth, i noticed it 3 days ago but it wasnt bad. ive only been with one person my whole life, and if it is herpes, does that mean he cheated? i dont know much about herpes, ive read that theyre usually clusters and this isnt, im also a bit of a hypochondriac but this really scares me. i dont have insurance or a primary doctor, does this look like herpes? image