Why cant I keep an erection?

I am a 20 year old male who is very sexualy active. I just had a tonsillectomy two weeks ago and have been off of pain meds for a week now. I thought that might have been the issue ( i read an articale saying the percocet reduce sexual performance), but i tried last night and still could not keep an erection. the girl isnt my normal sex partner, but one of the most attractive girls i have ever had intercourse with. should i resort to sexual male enhancments of some sort, or will my body get back to normal soon, because i had never had this issue before and it is bery emarrasing for a 20 yr old college student who is very active.