What Causes Vaginal Spotting And Abdominal Pain?

hi, i began birth control pills on the first day of bleeding in my last period, however, i didnt have a period but just spotted for about 5 days, since the completion of that box of pills, which was last thursday, i started spotting and was having immense pain, today the pain was almost unbearable, and then i passed a 'sack-like' tissue. could i have been pregnant or was that formed because i didnt hhave a period last month, and secondly, i am supposed to begin my new packet pf pills today, and im afraid to, should i or should i allow this period to flow first?