2 +test 4days afters missed period, sore boobs. experiencing somewhat heavy bleeding w/mild cramps 3days later even passed tissue. what could this be?

Miscarriage?: You may have had an early miscarriage. Up to 1 in 4 conceptions result in a miscarriage, usually because of chromosomal abnormalities. By itself, it doesn't mean you will have future trouble with pregnancies. However, it's important that you see a doctor to be sure there aren't any complications that could jeopardize your health or future fertility.