If the pressure is going down, you're losing water somewhere.
If you have a main shut-off valve on your tank (usually between tank and rest of house) shut that:
- If the pressure keeps going down, it's probably the check valve (submersible) or foot valve (jet pump), but could also be a leak in the pipe either going to or in the well.
- If the pressure drop stops, there's a tap open or leak in your house somewhere.
Alternatives to a main shut-off valve are certain types of bypass valves on softeners, carbon or iron filters (the type with two valves to turn -- if you just turn one, it shuts off the water), or cartridge filters are often installed with shut-offs so you can service them.
Diagnosing interior leak
Unless you can isolate the problem to your well, go around your whole house and shut all valves (including toilets and outside taps) and inspect for any leaks.
The time of year is also a hint that you might have had a frozen pipe that burst, and has now melted and is now leaking, so you should definitely take a close look at areas near pipes on/near exterior walls and at any outside hose bibs.
Diagnosing leak in well
Almost certainly this will involve pulling the pump/foot valve out of the well. You'll have to turn off the pump and drain the pressure first.
Before you pull it, see if you can spot an obvious leak above the water line (eg, at the pitless adapter at the top). Pro tip: use a good spot-type flashlight or a mirror to reflect sunlight so you can see down.
Once you pull the pump out, inspect all the fittings for any signs of damage. By far the most common problem is the check valve at the top of the submersible pump sticking open (either spring or other parts corroded, or something actually lodged in it holding it open).
If you still haven't found anything, this starts pointing at a leak in the line going to the house, which is significantly harder to deal with (involves digging), and in worst case, involves digging the entire line up. In my experience, this will usually be:
- Pipe was above frost line, and froze. If you know a spot the pipe isn't buried deeply, this is a good place to start.
- Lightning strike. Lighting will travel across trees/fences/etc to get to the water/power lines on its way to that giant metal casing in the ground. Usually where it hits it'll burst the pipe open, and it'll following the most direct path possible.
- Bad/broken fitting. There should only be one fitting undergound, which is at the pitless adapter where the pipe meets the casing. It's possible there's another at the entrance to the house, but it's best practice to just run lines inside and do the connections inside. It's also possible there's a coupler somewhere in the middle of the line if the installer was being lazy/cheap.