Can really tensing the stomach muscles when doing a pelvic tilt cause a disk bulge? hope its not a silly question, just when doing them my foot buzzes

Not a dumb question: Doing this exercise will not cause a disk bulge per se but will cause narrowing of the area of the spine where nerve roots are located and impingement of this would explain the leg pain/foot numbness.

May bring on symptom: Although not likely to cause a disk herniation or bulge, this action, called a Valsalva maneuver, can cause a very slight temporary increase in an existing bulge, and is a test used to evaluate patients for nerve root compression. If your nerve root is only irritated by Valsalva, then physical therapy or antiinflammatory medications are probably all you need. Ask your doctor for guidance.