I normally wear size S, ordered M for more relaxed fit, but it fits like XS. And no, I haven’t suddenly gained like 15 lbs overnight 
I also ordered another Alternative shirt in M, and it fits just as supposed: a bit loose, not snug at all. I have other shirts by this brand, and never had any issues with sizing. So, I’d say, this shirt runs about two sizes small. It’s not very long either, but not cropped. My guess is, this shirt is in junior’s, not women’s sizing.
Overall, it’s a cute shirt, soft and good quality, just order at least a size or two larger.
Rating: 4 / 5