It's hard to answer where it came from since I don't know what kind of climate you live in - these type of rashes are common in hot, humid climates. It is extremely likely that it could have come from the antibiotics he was given. Black walnut hull tincture (in it's black state) is anti-fungal, but can burn, so if you have a cream that is helping, I would stick with it. Whenever you can, take his diaper off and let him just air out - this will help to clear up the rash - fungal growths like warm, wet, dark environments. Keep him clean, give him lots of fluids and it should pass quickly.