If you want piece of mind you can see if your vet has ultra sound and have them do an ultra sound on her. That way if she is gravid it ultra sound will not hurt the developing embryos like x-ray could. Also a pregnancy test is normally a fairly inexpensive procedure that people do for their dogs and cats. If your vet does not have an ultrasound machiene they should be able to refer you to a vet that does have one.