Hi, I can understand your concern for painful lump deep near the muscle, tendon on arm between wrist and elbow. There can be a possibility of either a sebaceous cyst or a lipoma in your case. You should meet your treating doctor to get proper diagnosis. Do not try to even touch or pinch the area as can cause more aggravation of the problem and spread of the infection. This area is quite common to get infected very easily because of sweating and high bacterial proliferation. For time being period you can apply antibiotic cream such as gentamycin or fucidin or mupirocin and take pain killer tablets such diclofenac or paracetamol. Take care.