Family physician here - I'd suggest messaging your provider prior to the appointment. It's nice to get a heads up for what to expect prior to a visit. The billing side is tricky, and provider dependent. Technically, addressing a separate problem-focused issue at a wellness visit should result in an additional charge. However, I will rarely add extra charges unless the issue is really time consuming or complex (e.g. new chest pain), and even then I try not to for the patient's sake, but some providers do, and they would not be wrong to do so.
As another poster mentioned, depending on the issue, a PT may be more useful in giving therapeutic advice, but I disagree in that you should be able to trust your PCP to diagnose an acute musculoskeletal issue. Bear in mind PT visits can be quite expensive also, so if you already have the physical with your physician, it's worth trying to have them take a look at that visit.
Hope this helps.
