Two possibilities: First, something you have plugged in actually has developed an internal fault, and is leaking current to ground. If so, this is potentially dangerous, so fix the appliance. Most likely problem.
Second, the GFI/RCD is defective, and is tripping when it shouldn't. Since I can buy a new one for <$10, I would just replace it to see if that solves the problem. If not, go back to #1, above. A bad GFI/RCD is much less likely than #1...