Are Pectoral Muscles Actually Cut During Bilateral Mastectomy With Lymph

Are pectoral muscles actually cut during bilateral mastectomy with lymph node removal?
I had this surgery, with no reconstruction, 4 months ago and externally I appear healed and have my range of movement and strength back. However, my pecs near my shoulders are still sore and revolt if I do too much driving, too much heavy lifting, or weightbearing with my arms (as would happen in a yoga pose like downward facing dog.) I am wondering exactly why those muscles are still a problem: whether they were cut and sutured during surgery or just injured, or if it is nerve damage alone that causes the pain. I had 11 lymph nodes and surrounding tissue removed from my left axilla, and although the left pecs are worse than the right, both sides get awfully sore and somewhat painful when I "overdo" things. The area of my middle chest and incision line do not hurt like the area near my shoulders in front. I think knowing why would help me. Thanks.