4yrs post op breast implants, 10days ago whilst doing tricep dip exercise I felt a pop feeling on my sternum. What have I done?

I agree that you probably did no harm to yourself, the implants, or to the eventual outcome.  Exercising after Breast Aug is generally a good thing.  You need to listen to your body to prevent overdoing itIn this case your body is telling you to take it slowGood luck

You may have a muscle strain however the history of a pop suggest the possibility of a tear in the breast implant capsule.I suggest you revisit your plastic surgeon as soon as you can make an appointment.