8 years Breast Augmentation postop and left breast is swollen after muscle strain - what could this be?

I think you may well have answered your own question. It's likely that you have caused a little tear in the capsule of the implant while exercising as the capsule is stuck to the underlying/overlying muscle which has bled a little. You can use ice packs and a good support bra with anti-inflammatory medications while things settle down over the next two weeks. If the swelling/hardness remains or worsens after this time then see a plastic surgeon to ensure nothing else is amiss. good luck

You may have torn some scar tissue or capsule in your breast and now have excess fluid. I would suggest seeing a plastic surgeon and getting an ultrasound.

It’s hard to say, but you may have torn scar tissue or capsule during your exercise. If there was bleeding associated with the injury, it could be just fluid around the implant or a capsular contraction starting.
Return to your plastic surgeon for evaluation.
Best wishes,
Dr. Taranow