The only sensible thing to do is go and see your doctor. Not sure why you haven't done this already but maybe it is embarrassment or a worry that they might actually find something wrong...
If it is the former then believe me there is nothing embarrassing about it, frankly it makes no difference to the doc if it is your ears or any other part of your body that you are concerned about.
If it is the latter then if there is genuinely something wrong then the longer you wait, the worse you might be making things by not knowing the cause.
Chances are it is nothing major.
And as for the GF, just talk to her about it, this will take the pressure out of any situation before it arises. The last thing you need is to be worrying about how you will perform before it actually happens, thinking about not getting an erection is a major cause of not actually getting one!
Good luck bud.