What Is The Irregular Vaginal Bleeding After Taking Medication For Urine Infection?

i had a urine infection and was prescribed Trimethoprim which i took for three days as told, now i have started bleeding vaginally even though my period was 10 days ago. I am 44 , 5ft, 2", 65kg. I have two children one born by caesarean 14 years ago with no after problems and appendicits at age 8