What Causes Irregular Periods?

I have beeen having a weird cycle for the past two months. In September, my cycle changed from the regular 28 day cycle to 32 day cycle. Then I didn't see my period in October , I took two pregnancy test and they were negative and finally it came on the 8th of November(a 41 day cycle?!). Now 32 days has passed and my period hasn't started for December. What could be causing this chnage? Could I be pregnant? NB. Normally my menses would last for 5-7 days however the last one only lasted for 3 days