I'm 20 years old and i've always had irregular periods. i've lost 75lbs and my periods have gotten a lot lighter. could the weight loss cause this?

It is possible.: The main reason for light periods is change in hormone levels. Causes for what seems to be a light period can include: thyroid problems, eating disorders, excessive exercise, initial periods for a girl, use of an intrauterine device, oral contraceptives, perimenopause & implantation bleeding in early pregnancy which rarely occurs.