Welcome to gyne.org, johny.
I don't see obvious gynecomastia, but that doesn't mean you aren't in the beginning stages of an active phase. Something is definitely going on. If you can feel something under your nipples and it is painful, that is a classic sign of active gyne. It means your tissues are being stimulated to grow. The pain you are feeling is really no different than when a young girl starts to bud and have pain. It's the same process and hormone related causes.
You should look into WHY you are having this situation. Your primary care doctor or an endocrinologist needs to check your hormone levels and treat them, if there's a problem. Frequently, however, there IS no obvious reason for gynecomastia.
There are meds to deal with the pain, and they are most often effective in early stage gynecomastia....the kind you have. The med that is usually given for the pain of early gyne is called Tamoxifen. If given early enough, it can sometimes reverse the gyne process. It is especially effective in dealing with the pain of early gyne. Endocrinologists are usually the ones to prescribe a medicine like this; primary care doctors usually won't do it.
Curiously, I developed my gyne after beginning an exercise (weight) program, too. My son has experienced the same thing....on two occasions. He got rid of his by using the Tamoxifen and it did not come back.
I know exercise is not supposed to be a known cause of gyne, but at the same time, I continue to see numerous reports from people that their gyne started shortly after they began an exercise program (particularly weights). I know this was certainly the case for me and my son. I personally think there IS a correlation, but as far as I know, there is nothing in the literature that would support such a thing.
One thing we do know.....exercise may help with fatty gyne, when the breast is mainly enlarged because the person is overweight or obese. But if there is actual tissue growth, no amount of exercise will make it go away. Surgery is the only option for dealing with it, but surgery is only a viable option for gyne that is stable, not in an active state (as yours seems to be).
I don't think you are anywhere close to needing surgery yet, however. From what you've described, I think you are in the early stages of active gyne, and I think a trip to an endocrinologist is in order. An estrogen inhibitor or anti-estrogen med (Tamoxifen, Arimidex) could potentially be very beneficial for you before any tissue growth really starts to show. You could nip the problem in the bud, so to speak.
These meds are not without known side effects, however. You should discuss them with your doctor and decide if they might be right for you. They are typically given only short term (2-4 months).