It could be depression, it could be caused by meds, or it could be another issue entirely. It is something you should bring up with your doctor because it can be a huge issue in a relationship. I would also suggest that you bring your girlfriend along to see your T or pdoc when you discuss this issue so that she can hear from someone else that it's not her fault. This is something that a significant other can internalize especially if they have their own issues they are dealing with.