Hi Marie,
Pretty sure your multiport valve has a problem. Probably that 'spyder' gasket. If it's draining back air is getting in and that's most likely from the waste line. When the pump is running have any water coming out that waste line? Also to verify that's where the air is getting in can you cap it and see if the water drains when the pump is turned off? Doesn;t take much of an air leak for this to happen. What's the model number of the filter and valve?
Al
Edit: Just curious if while you were away if anyone else was taking care of the pool and operated that valve. If they tried to change positions with the pump running that would be a good reason for the gasket to get screwed up.