Implantation Bleeding?

I’ve been off BC for 3 years now and my BF and I have been using the pullout method with great success. I am very regular and never have any inconsistencies with my period. I have 3 children who are all older now it’s been 10 years since I’ve been pregnant but each time I have never had breakthrough bleeding just a missed period. This month my period was 4 days late which is not normal and then yesterday I started bleeding but it’s very light and it was pink yesterday brown and tapering off today. I can smell everything and if it’s not a good smell I get nauseous. I have taken like 6 pregnancy tests and their all negative. I feel like I’m going crazy here. I’m almost 40 and was feeling a little invincible perhaps with my birth control methods. What are the chances I am actually pregnant here!?!?! Thoughts???!!!!