Aschleman is right. One of the most common side-effects of mood-altering meds is erectile dysfunction. Check with the doc. There are tons of differing meds they can offer, juggle the dosage...that sort of thing.

A few questions though:

1) How old are you?

2) Do you exercise?

3) Get enough sleep?

4) Are you drinking with the meds? (a serious NO on anti-depressives)

5) And is this on your mind when you are with your girl? Believe me, you can and will psych yourself right out if you're thinking about it too much. It's like when you have a cough from an allergy. The more you start thinking about it, the more you cough. Same thing here.

6) Are things okay in your relationship with her? A lot of times, this can manifest itself as a pysch symptom when there is something you just can't bring yourself to address another situation.

Sorry to sound like I'm grilling you here but these are some things you might want to think about when you get in there to talk to your Doc.

Good luck.