Leukemia most likely.. But seriously come on ! this is not even related to what leukemia CAN do. Leukemia is the LEAST likely thing to cause this.
I understand that health anxiety can cause you think that minor cut will become necrosis ( dont google if you dont know what it is ) Gingivitis ( gum disease ) is 99,99999999999999999999999999999999 % the cause its very common, and not dangerous, but i suggest you still treat it, because it can be annoying and ruin your teeth. There are special toothpastes that protect your gums, and plus you may have sensitive gums, so soft bristles may help too... And hopefully you wont think that its something deadly, gingivitis is not even close to deadly !