Severely decreased sex drive at 20?

I am a 20 year old male in Vancouver, Canada. I am a first year Uni student without stress and have learned how to cope with stresses too. I am in a relationship with a girl I have met during studies, who I am very attracted to and have been for quite some time now.

I am not overweight by any means and I work out 3-4 days a week, 6' 0'', 175lbs at 11% body fat and I am in great shape despite the fact I smoke (only a pack every 4-5 days if even that) for the last year and a half. I take my omega 3s with added coQ10 and resveratrol, magnesium, b100 complex, multi vitamin and vitamin D. My diet is on average quite healthy, in the sense that I eat decently well staying hydrated all day and I feel and am healthy overall (except for the smoking part, don't shoot me ).

*PRESCRIPTIONS: - Dexedrine spansule 10mgx3 when I wake up prescribedbfor minor adult ADHD since late February 2012
- Clobetasol topical steroid for contact dermatitis on fingers on rare occasion since summer 2010.

So here's the kicker: my sex drive is in the tubes. A couple months ago I was always into it, would finish on time within around 15-25 minutes and even raring' to go a short 10 minutes after. Since a few weeks ago, even with foreplay my sex drive is pretty damn low. For example today we had started getting hot and heavy at my house (to be blunt we became very horny) and started to have intercourse. Part way through it, about 5-15 minutes in, I seemed to have lost interest. We went for an hour, she managed to get off a lot, but I had next to no pleasure. By the end I was drenched in sweat, had become dehydrated and it had all snowballed. I just became limp during the act after about 50min-1 hour. I had become ashamed, as this has normally never happened.

In summary: as of within the last month I have experienced a severe drop in libido, but have never ever had problems as I am a healthy, active 20 year old male. I was wondering too if the Dexedrine could be be having a side effect on my libido level.
Please help!