Thanks for reaching out. It's not uncommon for the pec muscle to get stretched or strained doing something, especially during the first year or so, while your body is still recovering. Of course, there are other potential explanations as well, like capsular contracture. But capsular contracture usually starts as tightness, and uneven breasts and then over a long time, evolves into pain. It's most likely to be the muscle thing and it's very likely to go away with time. Ibuprofen, if you can take it, might be useful for the next week or so to decrease the inflammation in the muscle as well.
All my best,
Dr. Jeremy Pyle