Could be a change in your diet or if you are exercising a lot. Also stress will play havoc with your period, you could be low in iron. I'd go to my local doc and discuss these issues , ask for a second scan and a blood test for pregnancy from doc too.
This happened to my cousin, & the doctors put her on birth control pills to regulate her periods. She only had hers once every three months & then it was like a river flowing. So you might want to ask your doctor about birth control pills to regulate yours.