I have backwashed sand filter and now it keeps shooting water out is this normal?

Have you been rotating the valve handle counterclockwise? It must be rotated clockwise. First turn off the pump, try rotating the valve clockwise a few times finally placing the lever in the "filter" position. Then turn the pump back on. Depending on the age of the piece of equipment it is possible for the gasket to have moved out of it's slot. There are about two remedies for this. Add a PVC ball valve to the waste line at a convenient location. You then have to remember to open and close the valve each time you back wash ~ not the best situation.

The next would be to replace the valve itself. But first try the first suggestion a few times. You might also want to try the backwashing technique a few times to see if it improves. Rotating the valve to the B/W position then to rinse then to filter then to B/W etc. There could very well be an obstruction in the valve seat. You may even want to try to remove the valve to inspect it for damage etc. k