http://lowerstomachexercises.com/wp-content/uploads/2011/08/seal-stomach-stretching.jpg
When I do this stretch my legs go numb after maybe 15-20 seconds. Should I be worried? This just started happening, I always do this stretch.

Well, first of all, are you breathing? Make sure you aren't holding your breath during stretches. It is strange thought that this has just started happening. Have you had any injuries or accidents recently?

That specific stretch collapses the lower back and may be causing circulation problems. As a substitute try cobra pose.

http://www.bikram-yoga-noosa-australia.com/Bhujangasana-photo1.htm

Cobra pose is sometimes extended into the stretch that you are doing, but at a lower level it is more of a baby-backbend. When stretching don't tense your arms, shoulders, or neck. That can also restrict the blood flow.

If the problem persists, talk to a doctor. Best of luck!