I thought I'd try one of the underworks $20 binders and got it in small. Well, it does absolutely nothing. Maybe if I had a huge chest or something it might do something. I don't know. I actually look worse with that on than just a sports bra :(
I'm never going to use it so if anyone else wants to try it pm me your address.
Exchange it!
Underworks has an excellent exchange policy. Return it and ask for an XS size.
Honestly, I really hate "tight" things around my middle and I think the XS might give me that "I can't breathe feeling". I feel like I'm eternally stuck at binderless.
Binders aren't the most comfortable of things, but if it's so tight it hurts and is hard to breathe then it's too tight and dangerous for you.