Why am I different sizes all the time?

Does anyone else feel like their pants size changes all the time? I have a pair of size 4 jeans that fit perfectly (though they have stretched out a lot). I normally fit perfectly into size 6 pants, but today while trying some on in a store, the size 6s were very tight. I tried on an 8 and that felt much more comfortable, but a bit loose.

I've never had to try on anything more than a 6 before....normally, a 6 is fine for me. I just don't understand what size I actually am.