Erectile dysfunction. Dysfunction?

I am a 26 years old male, I and my Fiancée were trying to have sex yesterday in car but could not do so properly as there was a little space in car also I could feel election in my penis but whenever she came above me and When I was not able to put the same inside her then the erection was lost and I had to shake it again, few times my penis did go inside her but as it was her first time ever, so she took the same out as it was hurting her a lot and so she took it out. But I am a lil worried as I was not able to maintain erection. Can this be a problem of Erectile dysfunction? I would also like to inform that I was completely exhausted at that time due to much physical activity during the day time, also I was quite nervous as we were in a car and was fearing if anybody could see us and also a day before I had taken a anti depressant tablet which caused a lot if drowsiness then. Was that a temporary problem or it can be a symptom of serve tile dysfunction. Kindly advise me I am nervous.