What Causes Dark Brown Vaginal Spotting While On Microgynon?

ive been on microgynon for 5years i recently had a 1 month break, in this time i had lots of unprotected sex. i have had a natural period and started taking the pill again but im having spotting on 3 seperate occasions now. buts its very dark brown. not like a period. i have done pregnancy tests but been negative. can i still be pregnant?