Sorry to hear about your problem. Pain at the drain site is quite common, specially if it is kept for a prolonged period of time (as in your case). Radiation
therapy can aggravate this pain and it usually subsides when the treatment is completed.Costochondritis
is unlikely in your case. In costochondritis the pain is more medially situated and it is episodic and not continuous.
You can use some over the counter analgesic
to take care of the excessive pain.
Hoping for the best.