What Causes Delayed Period While UPT Is Negative?

i was 7 days late for my period, i noticed that i started spotting so just assumed that my period was starting. However it stopped for a day then i began to pass several clots with cramping for a day. Then the next day there was no blood. what's happening now is just every now and then i get a little bit of blood on the tissue when i go to the toilet, but still getting little cramps. normally my periods are regular and are of a heavy flow but this month ive never experienced one like it. I did a pregnancy test the day i was due and it came back negative.