Was she kept with other hamsters in the pet shop, did they attempt to sex them? I know lots of pet shops refuse to guarantee sex (of the hamster!) but some will at least try.
I'm sure someone who breeds will be able to answer your question better than I can
but the only sure-fire way may be just waiting 10 more days and seeing. A swollen abdomen and more prominent nipples might be clues, and perhaps some behavioural changes, but as you haven't had her very long these might be difficult to spot and differentiate from just "new hamster" behaviour.
Sorry I can't be of more help
although one hamster I got from a pet shop a few years ago, we were convinced she was pregnant from the rate she grew outwards but not upwards, but she turned out just to be greedy, not pregnant! You won't have long to find out either way though.
Emma x