Here's why it happens after sitting a few days. The torque converter is full of fluid. Most of that fluid "sits" high. Higher than the fluid level of the trans. Over time that fluid "drains back" into the pan. Making the fluid level overfull.
The new O-ring on the fill tube should have fixed it but its possible you cut the new O-ring or maybe the fill tube has some damage.
Excessive drainback could be an indication of wear in the trans.