This happened to me before. Turned out that my GP had somehow been registered with the insurance company as a specialist, which required a higher copay. No idea if that's the case with you or not -- certainly my doctor's office knew about it, as every patient with the same insurance company as me was having the same problem. Eventually it got fixed, and the doctor's office floated the extra money until it was.

I guess my point is, ask your doctor's office about it if you haven't already.