I have a bruise-like pain under right breast 4 years after silicone breast implants, under muscle. Any ideas?

I'm
sorry to read about your discomfort. It really is hard to know what your pain
is due to. I would definitely return to your surgeon for an examination, and to
be sure there is nothing wrong with the implant you may need to get an MRI
scan. It is not likely but it is conceivable that a small leak of the implant
is causing inflammation and soreness. A pulled muscle can cause prolonged
soreness, but that should eventually get better. With 3 months of constant
pain, it is important to get it fully evaluated. Best wishes.

This far out from surgery it may be a capsular contracture.  Capsules can hurt if they are tight enough.  This needs to be evaluated to determine the cause of the pain.  It must not be ignored.

I think that if you are having consistent muscle pain, then it is best to be seen in person by your surgeon to be properly evaluated.