I find it hard to ejaculate during sex recently.?

I have been with my GF for 4 years and I we have sex almost everyday but never any longer than 2 days between. Well I have always ejaculated with no problems. The last 4 times though I have not been able to. I even tried masturbating and I still can't ejaculate. I am 22 and she is 21 and it still feels great actually having sex and I stay hard and it is a big turn on, but I just can't ejaculate when it comes down to it.

I just got over the flu and have been taking ColdFX and robitussen Dry cough tabs and tylenol extra strength and vitamin C. Could that have anything to do with it?

We also just started a work out routine 3 weeks before I got sick and I stopped drinking anything except water(but still eat the same). Could that have anything to do with it?

I heard that ejaculate has alot of protein in it and I was also wondering if I don't get enough protein because I am working out alot and not taking in enough, if that could affect my ejaculate.