mastitis

i have been having regular cases of mastitis now every month for 5 months, this normally occurs when due or on my period. The strange thing about this is i have not breastfeed my son in 2 years and this has come out of the blue. i have regulary seen my G.P who keep putting me on antibotics, but this is only helping the problem at the time not stopping it. if you could help me in this department, or if any one has had this problem please email me.
thank you