Small Rash appears on arm after nights out

Patient: Hi, For a couple of years now after nights out, the following day i get two small rashes/lumps on my arm in the same two places. Both on my right arm. it isnt painful but can itch a little bit, the rash goes down throughout the day. the rash is small red, and looks like a nettle sting, what could this be?

Symptoms: Small rash on the arm, a little itchy