Pants don't fit already?!

I'm 8wks 4 days and my pants don't fit. They hurt to wear them. All I can wear is extremely stretchy waisted pants or maternity. It started about 7 wks pregnant. This is my 4th pregnancy but 3rd baby. I'm not gaining weight if anything I lost 2lbs. It so bad I'm starting to think I maybe pregnant with twins because I know my dates aren't wrong. What's going on? Anyone else like this?