[souffle]

Normal pregnancy lasts 16 days. If they go on longer then there is a much higher chance of the pups being born dead and over 18 days I would see a vet.
The fact she is acting oddly is a bit concerning. After the pups arrive the mothers do stay in the nest all the time practically to tend the babies. Are you sure she hasn't had them.
After this time she does need checked though so I would encourage her out with some babyfood or tasty veg and use a clean spoon rubbed in the pee corner to part the nest and check for pups. If she is still very swollen there could be dead pups inside her and this is a dangerous situation. Having pups at this age is also not recommended as the mothers can have delivery problems but accidents do happen so you just have to do what is best for her. Older hamsters are also more likely to have a womb infection after mating which can cause swelling.
Is there any odd smell about her or any discharge? Is there blood on the bedding?