I'm on nexplanon (2yrs) and i've had extremely sore and swollen breasts for the past three weeks. never happened before. what is going on?

Hormonal changes: resulting from the Nexplanon and/or any other type of hormonal birth control can cause swelling and/or soreness of your breasts. NSAIDs (eg. ibuprofen (Advil) or naproxen (Aleve) should provide some relief, but taking these medications frequently on a regular basis may not be a good idea. Depending on how long you have had the Nexplanon, this is a side effect that may resolve in a few months.