I just put a whole house filter on my well. While doing so, I completely disconnected my well, reconnected, and primed it. But, I can only get 35psi. I originally had 45psi. Natually, it runs until I unplug it. How did I lose that 10psi? Do I just need to pump up the expansion tank?

Does that new filter have a bypass, if so engage it to see if the filter isn't throttling down the volume too much. Other folks have confused volume and pressure when they took their readings. Can you tell us how and where you got those reading and were any spigots open?