Hi, thanks for sharing your health concerns with HCM! If I were your treating Doctor for this case of pus-filled lump, I would come up with one possibility that is some infection! However, the mode of infection could be some mild penetrating injury, insect bites, folliculitis or small boil! Therefore, you need to apply ice/cold compress and some gentamycin-like antibiotic cream and if not relieving, swift on to oral erythromycin 500 mg four times for 5 days! Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions. Wishing you good health.