I m suffering from extremely bad sudden onset heartburn whilst taking the second days course of fluoxcicillin for mastitis. It s feels extremely painful very high up in my throat and not like feelings of heart burn I ve suffered before. Much more intense and localised. It came on about 30 minutes after taking last dose. My symptoms of mastitis had very much settled down even before starting the course as it was quite a mild case. cam I stop the course as this is extremely painful and I cannot sleep.