could be a bad coolant sensor, which is inside the overflow tank. sometimes they get stuck, but here is what i would do before i replace anything.
let the car sit overnight, tomorrow morning turn on the car, take the radiator cap off, and fill it up to where its supposed to be. with the car on, the coolant is passing thru the block already so it will be a true level reading. i had it do that to my car when i flushed the system. i checked it and it was fine, turned it on and the low-level light turned on, turns out all the coolant in the overflow tank got sucked into the engine, so i just had to keep re-filling it till it was full. total i had to re-fill it after i thought it was full about 3 times.
RIP Gloria A. Torres
4/26/61 - 10/30/07
The best Mom-lady I could have ever asked for