Allergy Shot Sites Itch After Taking Ibuprofen

Patient: I have been taking allergy shots for over 3 years. For some reason, on the odd occasion I take an Ibuprofen for a headache, my old injection sites in about 8 places swell about 1 cm wide for a few hours. There are no rashes on any other parts of my body after taking the drug and no allergic symptoms so far as I can remember. Does Ibuprofen trigger a release of histamine in those spots? What is the reaction taking place between the Ibuprofen and my allergy serum still under my skin?Thanks,