Sudden change in menstrual cycles is certainly due to some abnormality. It is right to first check for pregnancy
and completely rule it out before assuming any other condition to be the cause.
Some of the most common causes have been ruled out with the investigations conducted, but there are a few more which will require a check on your hormone levels, a urine analysis, vaginal swab test and pap smear
to rule out/diagnose other conditions.
Hope this helps.