Why am I having painful muscle cramps in my breast?

Probably not cramps. The breast tissue is made up of glands, fibrous support, and fatty tissue, but not muscle. There are muscles underlying the breasts that can get sore with overuse or strain, however.

Related Questions

I've been having muscle cramps into the left side of my chest/left breast. Are these coronary muscle spasms?

Unlikely! Does not sound like typical heart pain but one should have more history (age, duration, circumstances of the production of the pain, other medical conditions etc!).
No. At your age, very unlikely to have coronary muscle spasm. Your heart is normal, unless I am not aware of a heart condition. However most likely this is an intercostal or rib pain in the area. Also could be do to muscle spasm as well.