What Causes Irregular Vaginal Spotting While On Birth Control?

Hi im 19 years old and last month on the 31st i found out i have a cyst and that day i also got my period. .. i restarted my birth control after a month of being off of it on jan.4th ... on jan. 15th i spotted and it was very light and stopped now today on jan.20th im spotting again and it seems to be going away and yes i am sexually active