Replying to SLight problem

Last night is what started off as a great night turned into a nightmare..I was hangin out with this girl..and we started messing around. I took her shirt off and she proceeded to take my pants off, i was like alright sweet Im gonna get some head. So im harder than diamond at this point, she starts to suck on it and does so for like 10 mins. Somehow with only taking ONE like 5 sec break. Other than that he mouth never left it. Now she doesnt deep throat or anything but after about 10 mins i started to go soft and I had no idea why. She stopped after that and I couldnt get a boner after that. PLease god someone help me Im about to go to the doc and request some viagra