Most likely it's the GFCI breaker gone bad. Has anything changed that would cause the pump to run differently? Anything else powered off the breaker? Does your pump look like it's seen better days? Even if looks trashed but runs it's probably not the pump.
For the past 3 years my pump would rust up and would not turn till i broke it free. The bottom of the motor was rusted away. But it still ran. Even with what looked like a death trap it was the GFCI breaker. I have two circuits that I can plug my equipement into so a quick swap of the plugs and let it run for a couple of weeks with no issues basically confirmed the issue.
If you are not comfortable/capable swapping it, have a professional handle it. It's an important safety device.