Ouch! Something is plumbed wrong or assembled wrong!

What brand and model of pump, filter and multivalve do you have, and what kind and size pool?

Your multivalve should have 3 openings large openings: From the pump, to the pool, and backwash waste. They should be marked on the valve. If they are correct it's possible the multivalve is assembled wrong, such as with the spider gasket incorrectly placed, or the wall between two chambers in the valve broken. Unless there's something esoteric going on, that's your most likely culprit. I'll let the folks who are more expert on multivalves give a better explanation