What Causes Irregularity In Periods And Nausea?

Hi. For the past 2 months, I have had very short periods. Normally my periods last about 4-5 days, they have only been lasting 2 days. I ve had random bouts of nausea. The periods have been heavy the first day and then gone or only spotting the second day. I m not on birth control, and I m concerned in this change. Could something serious be going on?