Now that I read this again, I left out some things. The pool is being fed off the 20 amp GFI breaker. When I turn on the shed light, the GFI trips (even if the pool pump is off), but the shed light stays on. Could it be that the shed & pool are on a 12-3 wire, sharing the nuetral @ when the switch is thrown the nuetral picks up a ground?

Assuming that it was all wired correctly, this is what to look for i.e. why breakers trip.
1. Overamp
2. Short
3. Water- since it is a GFI
4. Faulty breaker

If the breaker is indeed tripping, all components in the circuit should shut down. Since the light remains on I would guess that current is leaking through the breaker. Is the light as bright as it should be, or is it dimmer? You can verify all of the above with test equipment. If you are unfamiliar with testing procedures then you would have no choice but to call an electrician. Hope this helps.

thank you