Hello, Thanks for sharing your health concern with us. I have gone through your query and would like to offer my opinion. In the current scenario, chances of pregnancy seem to be less likely as you are very regular with your contraceptive pill. Weight changes that happen while on the pill can lead to hormone imbalance thereby leading to breakthrough bleeds. Implantation bleeding is a fresh bleeding seen around the expected period and lasts usually for a couple of days. Brownish spotting is suggestive of old bleeding. A trans-vaginal/pelvic sonogram is helpful in this situation. Hope this helps.