I also had pain in that part but then, I left badminton for a week and it was alright but don't know the reason...Can some1 tell?
I'd have thought the most likely causes would be tendonitis or maybe a mild muscle strain - either way, the best thing to do would be rest it (and obviously go see a doctor if it doesn't go away by itself).
Biceps tendon is there, true, but you can also have rotator cuff pain refer to there (which is a little more common in baddy players than bicep pain) See a physio or a doc to confirm. Most physiotherapists are better than GP's in this stuff.