GFCI circuit keeps tripping on electrical panel
I think you were on the right track by looking for recent changes:
I unplugged a heater which was really the only thing in the wall that was added in the last month. It didn't happen for a few weeks. Then it started again.
GFCI or AFCI?
While you mentioned GFCI, the breaker may actually be a combination GFCI + AFCI breaker. As I understand it, the type of damage from a high current device like a heater may actually be more in the nature of an arc fault than a ground fault. The solutions are largely the same - track down the damaged parts of the circuit and replace. But an arc fault as a cause seems more plausible to me.
It could be unrelated, but logically it may be due to damage caused by the heater's frequent tripping of the breaker. That could be anywhere, but the most likely places are:

The breaker

Nothing lasts forever. While I suspect the breaker is rated for thousands of cycles MTBF (mean time between failure), it may not really have handled so many trips very well. The problem is replacing it may not be so easy, and probably not recommended for you to do that yourself if you are not familiar with breaker panels. In any case, it would need to be with a proper compatible replacement - same or compatible brand, same type (including GFCI and/or AFCI), same current rating.

The outlet

The outlet and/or the wiring going into the outlet may have been been damaged in a way that caused the trips and now, due to repeated problems, it may have become the source of a new problem even without the heater. To check this, turn the breaker off and open up the outlet. If you see any burnt wire, melted insulation or other damage, or a backstab connection instead of screws, replace the outlet. Actually, I would just replace the outlet in any case - just get a standard 15A or 20A outlet matching the old outlet, and make sure to use the screw terminals.