What Causes Irregularity In Periods?

I have had my perioud for three weeks now going on my fourth. I am thirty years old. The first week was normal the past two weeks have been light flow like it is the end of my period. Last week there were large clots of blood one day. I took a pregnancy test today it was negative. In the past my periods have always seem to last about four days with very heavy flow and very painful for the first two days of menstration. I have not had any pain but it is really a scary feeling to not know. I know I should see a doctor and although I am afraid because I have never been to gyn., I will do what is needed. Any comments would be helpful in the meantime to ease my mind until doctor visit.