I'm 3 1/2 months pregnant, some lower back discomfort, performed a cat stretch the other day, now excruciating pain in lower lumbar. What happened?

See doctor. If you are truly in excruciating pain that has lasted several days, you should see a physician for a full evaluation. While it is possible you simply pulled a muscle, you might want to see your doctor to be sure there is nothing more serious going on.
Maybe a disc. It is unlikely that at this early stage that the pregnancy has much to do with this. It does cause your joints to be looser and that may explain this partly, but when you arch your back very hard in a forward direction, it does put tremendous pressure on the discs that separate the spinal bones from each other. Too much pressure can cause the discs to bulge and press on the spinal cord.