That was the first symptom i had which was diagnosed as sciatica almost 4 years ago, since then i have had a few operations and last september i had spinal cord stimulator implanted as the nerve is damaged.
See your doctor and get this looked into.
After seeing your doctor, go get a catalog -- depending on your size, you can use Roaman's, Woman Within, or Vermont Country Store, among others. They have underwear without elastic on the legs, and it may help solve the immediate problem (rather like the old 'pettipants' I used to wear as a kid, but in cotton) while you're worked up for the pain.
Hope you feel better!