WTF is going on with my periods? HELP

Erin
1st august 27- September 1 (first period)
2nd September 26-30 (normal amount of time between)
3rd November 6-10 (a few days later than expected)
4th January 4-9 (20 days later than expected)
5th February 25-March 6 (a little later and like 9 days long)
6th march 24-30 ( a week or so earlier than expected)
7th may 2-6 (a few days later than expected)
8th may 20-? (Two weeks early?!?!?)
Aren't periods supposed to become more regular over time? Not more IRREGULAR.
My periods are really heavy and painful. A couple months ago, I went to a medicenter because I had pain that started a couple days before that. This pain is the kind of pain that makes you stop everything you are doing and curl up into a ball. Anywho, the doctor ordered a urine test that came back with nothing. He then said to go to the hospital suspecting that it was appendicitis. My mom took me to the hospital, they did a blood test and it came back negative. ( not appendicitis ).
They sent me home and I still do not know what caused the pain. My sister has endometriosis but my mom claims that it comes from her exes side of the family. 
If you have any suggestions or ideas that would be helpful.