The filter is on the pressure side of the pump so no air would be coming in on that side. Has to be on the suction side. Most common place to look would be the pump basket lid. This is the most likely place to be disturbed and change. Clean the gasket and lube it with silicone grease. Next common air source could be a low water level and a vortex forming in the skimmer momentarily. If a vortex forms the pump will suck a little air, flow drops, vortex goes away and you may never catch this transient event. Also a sticking weir, either a door type or floater whichever you have. If the air source isn't any of these then the plumbing would be suspect next but it's too coincidental with you cleaning the filter so I doubt it's a plumbing leak. Keep in mind that when you cleaned the filter the flow probably increased so that makes me more suspect of something skimmer related.
Al
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