The overdrive light keeps flashing on my ford explorer. We changed fluid but it still flashes.?

We changed the transmission fluid and it went out for while. It didn't come on unless we went up a really big hill or mountain. Now it seems to come on all the time. This has been going on for a while. Couple of months. Rpm's are fine, as well as gas mileage. Transmission does not seem to be slipping. All gears are fine. No funny sounds or anything.