Hi, It is unlikely to be an allergic reaction from the antibiotics, due to the fact that the rash is not generalized (all over you body), and it only occurs on pressure (sitting down on buttock). I believe this is a rash as result of pressure on that area or friction burn. I would suggest to: i) Use of emollients of your choice, (any mild moisturizing cream would do) ii) Sitting on cushioned chair, avoid sitting for prolonged length of time iii) Increase fluids intake iv) Use cool flannel on the area to calm down the rash v) If it is itchy, try an antihistamine cream over the counter to help. Please do see your physician if no better after 1-2 weeks or seek help earlier if worse. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Joy Anosike, General & Family Physician