It sounds to me like you either have a plugged duct or possibly developed thrush from the antibiotics. Does it hurt during letdown? Burn at all?
For the mastitis and plugged duct I would put warm compresses on before nursing. I filled old socks up with rice and put them in the microwave to heat them. Nurse on the infected side first - that is when your child has the strongest suck - and perhaps try to massage the area that hurts.
If you obey all the rules, you miss all the fun. - Katharine Hepburn