This is just my opinion, but since the bleeding was very heavy and clotty and the blood test was neg. I really seriously doubt you are pregnant. If you hormones are off it can cause the period to be shorter and the other symptoms as well. I would wait and see what happens with this next period and go from there.
it is totally possible !!! pregnancy tests arent always right espesially if your still early !! it is very possible to have a false negitive. whats not possible to have a false positive