I have a question about pregnant dwarf hamsters.. please help?

hey i bought a pregnant dwarf hamster ... well the guy said it was pregnant and it was even separated from the rest in the pet store. then i placed her in an aquarium. then my brother said it has grown much bigger a week after i bought her. and i also notice that whenever i try to pick her up she seems to have a defensive behaviour... and its been three weeks and she still hasnt given birth.. then by the third week, i noticed her became more prominent and she did become quite bigger.. and i do notice some "nests" in the cage. but there are quite alot of burrows. but it has been more than twenty one days... is she really pregnant? or do you think she is?