HELP! Erection difficulty
Recently i've been having problems getting/maintaining an erection. I'm 21 years old so i know it can't be an age thing. I don't smoke, and rarely drink, plus i'm very devoted to health. Eating right/lifting/etc. A few months ago I could go forever in bed and orgasm a few times. Even after my g/f was tired i'd still be hard. Then at one point, I was taking some prescription cold medicine and began to notice it wasn't getting hard, so i attributed it to the fact that the medicine was doing something. I was also taking an anti-estrogen supplement and thought that maybe I had so much excess testosterone that it was going limp. So i stopped all that and even now a month later I find it difficult to get an erection. I was used to be horny 24/7 and now i kinda find myself just.... well... not. Its so bad that even sometimes when i'm in my GF it'll go limp.
I thought it could be a menthal thing to. That i was trying to hard to please her, so i began to really try and relax and if she went great, if not, oh well. However I know i'm 100% comfortable with her and at best I can go once and then its flacid for the rest of the night. Even the one time it is hard its not as hard as it used to be a few months prior.
So please guys, any suggestions? I shouldn't be going through this at 21.