Hi Bethany - radiated muscles as you know are a whole differen ball of wax from the regular ones - I find my muscles in my neck tighten when I exercise as well and they cramp. And cramped muscles swell (lactic acid build up etc) it's very possible the muscle is having trouble due of all of this including lymphedema when you over exert them. This lump might just be a part of the muscle that's spasmng (cramping) and then there's car tissue too. Try not to freak... Muscle do knot - radiated ones probably do it worse... But do try to move your appointment up to eas your mind. Also remeber radiation thins tissue out - this lump may have been there all along but only noticeable now - hugs!