I have a large knot in the bottom inside corner of my right pec that i've had for years. it's tight, and seems to be hindering muscle growth now as it is smaller than the other. is this fixable somehow and what may have caused it?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. It would be prudent to see a doctor. Start with your primary care provider to ascertain if a referral to a specialist may be needed. Wish you good health!

Scar tissue caused during workouts can cause the symptoms you are describing. Unfortunately so can other conditions such as a breast mass. (yes, they occur even in males.) Please have your mass evaluated in person by a Dr. Palpation can tell the Dr much about the location, connectedness and texture of the mass. You may need to have an ultrasound of the area as well.