So my period was nearly 3 weeks late, a took two HPTs ( 3 days a part, both with first morning urine) and both were negative. Yesterday I began bleeding like a period, only a little heavier and more severe cramps. I few minutes ago when I went to the bathroom I noticed a rather large blood clot ( very similar to the ones I had during my post pregnancy bleeding after I had my daughter 6 months ago). What would cause this?