Could i have herpes?

i could posibly have herpes because i had unprotected oral sex with my girl friend who has had oral encounters before. We have had sex but all contact was with a condom and she hadn't had sex before so there's little chance i got it that way. When i woke up yesterday i had small blister at the bottom on the head of my penis. It looks like it has fluid inside and in the middle there appears to be a darker substance. There is only the one small blister and it disapeared during the day to be completely unoticable. If i squeeze the head of my penis it becomes slightly noticable as fluid goes into it. But otherwise it can't be seen. It was clearly visible when i woke up today, again it has disapeared after a little while. Could this be herpes or do herpes blisters not go away like that?

I have been feeling quite run down lately with a heady feeling and blocked up nose but that could also have been caused by a heavy weekend or just general cold. The blister does not itch and isn't sore, apart from it's visible appearance i don't notice it.

The last oral sex occured approximately 12 days ago, but i have been having oral with the girl for about a month before.