O/D Off light flashing?

I am driving a 97 Mercury Mystique and as I was driving to work this morning and my transmission got up to 3200 RPM's before shifting to 2nd gear. then when it shifted to 3rd gear the O/D off light started flashing and then it started shifting like normal. I turned off the car and turned it back on and it did the same thing. The fluid level was a little low (toward the bottom of the ok zone) so probably isn't that. Any help will be appreciated...