We have just had the same problem in K-Mart with the mens clothing. Hubby2b was looking at new shorts and in 1 colour the size fitted perfectly, then tried another colour and the same size was too small. Then, tried the same colour again in that size and that one fitted.
Ally's right, don't let it get you down, maybe it was just a reject... the buttons might be slightly out, or they might have even put the wrong tag on it - and its actually a 14?? Just remember how good you feel because the 18s are too big
You must be doing something right!