What Causes Pink/brown Vaginal Spotting While Had Tubal Ligation?

i had mytubes cut last december and have had normal 3-4 day periods since however i had a nomal period kinda light but mostly normal this month and finishe abouta week and a half ago...today have started spotting light red,pink and brown spotting ih abdominal and lower back cramps? i ahve had unprotcted sex but didnt think it was a worry since i had a tubal ligation last year? any suggestns or ideas at all would help...thanks