I would guess that it is just a cold going around, but if you are scared, I would talk to my doctor. My big issue would be, why are both of you giving sex to the same guy. Maybe great for him but I wouldn't want to share that experience with just anyone.
Hmm..I'd say do what the person above me said. Or if for some reason you hang out with this 'best friend', maybe you just happened to catch a cold and then passed it to her, and then she did this?? Go to the doctor.
Honey, go to the doc and say you had oral sex and are afraid you have caught something from doing so. It's not the worst thing he has ever heard, in fact it is rather mild. At least you were practicing a kind of sex that doesn't result in an unwanted pregnancy.