I scrape what looks like a layer of white/yellow buildup on the bottom of my tooth (that had cavity in it) and it starts bleeding around the gums.why?

Dental plaque: Sorry kitten78961. I missed the second part of your question. Plaque is a mix of bacteria and the enzymes they secrete. Due to the sensitivity in this tooth you are trying to ovoid using this side, hence more dental plaque (bacteria) accumulation. Bacteria is a cause for the mild gum inflammation around your tooth and bleeding.

Dental plaque: The stuff you scraped off is dental plaque, food debris full of bacteria. This is the stuff that causes cavities.Removal by brushing and flossing will go a long way in keeping your mouth healthy. Get to a dentist and get your cavity fixed now. It will get bigger and deeper over time if left untreated. And this can cause more pain and the treatment more complex and costly.

Materia Alba: You scraped off materia alba which is a white cheeselike accumulation of food debris, microorganisms, desquamated epithelial cells, and blood cells deposited around the teeth at the gumline. This and the toxins that the bacteria excrete causes gum inflammation and ulceration which causes the gums to bleed. Brush and floss better. Have your dentist show you how. Get a professional cleaning.