What Causes Vaginal Spotting While On Tri-sprintec?

i am on tri-sprintec birth control, and my periods come on time very month, this month i have spotting two days before my period is due. i am on time everyday with my birth control, and never late. should i be worried this is implantation bleeding or is it just spotting before my period. The blood was bright red and rust red and alot like discharge but it is like that at the beggining of all of my periods. but it was light and stopped after an hour. i would like to know if i need to be worried or not.