Was My Aunt Actually Right?

While I was visiting my grandfather last week, my aunt made what I interpreted to be a snarky comment regarding my appearance. Specifically, "you're really thin. Did you do that on purpose?"
I should mention that my aunt is a size 6 and her daughter (ie my cousin) has giant breasts but yet no ass at all and is about as big around as my pinkie. I mean, I didn't think I was fat before. I got up to a whole size 6, and while I guess that is sort of big for someone only 5'1" it doesn't seem like something to comment on.

But I went shopping yesterday. And I bought a skirt. And I was a size...zero. Now, I don't think that grown women should be that size. So I'm a little freaked out. Since I didn't do it "on purpose" - I'm just really stressed and busy and I walk at least three miles every stupid pointless day. But still, this is scary. Nobody who actually sees me every day has said anything, just my mom and my sister (who always have something to say about my weight no matter what it is) and now my aunt. Should I be worried? Does size even mean anything, anyway?