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
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.