How To Treat Swelling Under Both The Armpits?

Hi, Im 42 and have for some time now had swelling under both armpits, I first put it down to the fact that I am over weight Im 5ft 2 and weight 12 stone, I have lost a stone and hoped they would go down, I assumed it was just excess fat, when I palpate the area, their are no obvious lumps, little tender perhaps, just as the breast tissue becomes on a montly basis. Hadnt been concerened at all about it affecting my health until I read something about lymphoma!! I have ulcerative colitis, since 2001, which does not affect me too often, I take 800mg of mesalizine daily