What medicine should I give?

Mastitis is the likely problem. Do not start any medicine blindly, because the wrong antibiotic can leave the quarter damaged and the milk unsafe. First milk out the affected quarter fully, keep the udder clean and dry, and do a CMT test if you can. If the quarter is hot, painful, or the milk has flakes or wateriness, she needs a vet to choose the right treatment today. Treating it now is far cheaper than losing that quarter’s milk for the whole lactation. Consult a veterinarian before administering any medicine or treatment.