I overstretched and felt a pop, and sharp pains under my right breast. Is this just a pulled muscle?

It is not possible to tell without an examination.You
need to be evaluated in person so the anatomy and available operations can be
presented to you. Then, you can consider what your goals and expectations
are. When the surgeon and you agree on a plan that takes into
consideration your present state, what you are trying to achieve, what is
possible, and what will make you happy, then you can proceed.

Thank you for your question.It would be very unusual for you to have caused any kind of damage from just stretching.  Still, if concerned, it's best to contact your surgeon to discuss the issue and possibly be evaluated in person.

The final appearance, shape, and movement are not exactly the same as normal breasts. The surgically enlarged breasts do not move in the same way as normal breasts. They tend to be firmer. The contours are usually somewhat different than normal breasts. In some patients these discrepancies may be rather noticeable. Although every effort is made to place the implants symmetrically, complete symmetry is rarely achieved. Immediately after surgery, the breasts are swollen and firmer. The final shape and size is approximated after 2 to 3 months, but up to one year may be required for the end result.

Thank you for your question.  It is difficult to cause damage to an implant and/or the tissue after a breast augmentation from stretching.  You may have pulled a muscle but that's not something to worry about.  Your right breast implant is settling into place which is something you want to happen.  It's always best to have your PS take a look for reassurance.