Dress Size ????

I have the honor of being a bridesmaid in a wedding for one of my best friends this coming March. My friend is trying to find reasonably priced bridesmaid dresses. Last night she called me and asked what my dress size was. I realized I have no idea what dress size I am. The last time I bought a nice dress was over 3 years and 2 babies ago. So I decided to look at some of the clothes I had to see what size they were. This is when I discovered something funny. The shorts that I had on and that fit nicely are size 6, but the jeans that I have been trying to get into for the past few months and can now finally, but barely, zip up are a size 8. This makes no sense. How can size 6 shorts fit better than size 8 jeans?