What Causes Irregular Periods?

hi i had my daughter 18months ago i went back on the pill for 6 motnhs then came off to try for number2 (8 months trying). i had my wiithdrawle bleed then a period missed a month then had a 102 day cycle then an 45 day then a 40day cycle iam know on cycle day 43 (3day late if haveing the same as last month) i have done a pregnancy test which was negative. what sould i do