Strange symptom - white lumpy “rash” along inner arm

I have had full node clearance and radio and have had episodes of mild swelling in my hand but seems under control.  However, I have another weird symptom related to the lymph issues which is that my inner arm comes up in white bumps in a sort of rash pattern if I overdo things. The skin becomes very sensitive too. My consultant is flummoxed. Anyone else had this or come across it before? Many thanks.