Hi,
Pain after breast augmentation is part of the process but it usually subsides pretty much in the first couple of weeks.
However, it is not uncommon for patients to complain of jabbing pains upon arm motions or workouts even for a more prolonged time. Numbness, stabbing pains intermittently, hypersensitivity etc....these are not the usual at 8 weeks but certainly warrant an opinion of your ps too make sure that all is well.
A capsule scar builds up around all implants and perhaps this has entrapped some of the small nerves that are giving you these sensations. If all is okay after the ps evaluates, then the recommendation will more than likely to be to give it more time. Tests can be done on a needed basis but time is always a good healer. If you are finding that the sensations are getting worse, then something may need to be done. If you are finding the symptoms getting more infrequent, then this is a good sign.