I recently had this problem with two of my African Clawed Frogs. I joined a yahoo group called Froggy Friends that is specifically for ACFs although they also help some with ADFs. They say that my frogs had strokes that were probably caused from some water quality issue. Perhaps this could be the case with your frog too. Mine also had problems swimming and had convulsions too. Some frogs are able to pull through although mine, unfortunately, did not.
Are you able to test your water? Are all your parameters ok? Make sure ammonia is 0, nitrite 0, nitrate below 40 ppm, and pH around 7.0. If not, you will want to take measures to get your parameters to these numbers to prevent your other froggie from becoming sick as well. Best of luck!