I had that same problem however unless you take her to the vet for an x-ray there is no real way to tell. Do you know when her last heat was? If she is an inside cat when is the last time she got out while in heat then count roughly 63 days from then I found a link on another thread that has a pregnancy calendar for cats http://www.purrinlot.com/preg.htm
that may help if you know when her heat was. Sorry I couldn't be of more help!!