H&M are often american sized, which can be misleading - at a size 20 I bought two jackets in there which were labelled a 16...which of course is an american 16, uk 20.
This is why I loved shopping in New York when I was a UK 16...buying size 12, fabulous.
Having said that H&M have a very cavalier attitude to sizing, sometimes its miles out in the other direction (eg buy a ten, fits a 16...)
Tbh I don't really care about the shop girls or their snotty attitudes, I'm only in there for a little while then I go away again and if they criticise me behind my back, well I don't know about it nor care!