What Causes A Heavy Period With Blood Clots?

i am 27 years old and i always have a normal monthly cycle that is approximately 27-28 days and 5 days of a period. this month my cycle was due on june 5th. on the june 1st i began to spot and was having lower back pain. i continued to bleed for about a week and a half with the 8th and 9th being my heaviest days. I stopped for a few days but on the 16th i began to have light bleeding by which the 21st began to get heavier with a lot of blood clots and since the 20th i have been having excrutiating back pain. what are the possibilities of what could be happening. i took a hpt yesterday that was negative