i think my mouse has gone crazy... seriously.?

so a while ago, we bought two baby mice and small cage for them. the base of the cage is about 1 ft. by 8 in. maybe four inches high, the upper level is about 6 in. by 8 in. i know its a small cage, but they were going to be eaten! anyhow, on to why i think my mouse may have gone off the deep end. so when we got them, they were both normal, you take them out of the cage, and theyd happily run around, sniffing and being mice. then, about two months after we got them, i noticed the smaller, younger one running in circles in her cage, counter clockwise. shed run around the bottom, up the stairs to the top down the tube to the bottom and left in a circle to the top again. she will do this four hours on end. i didnt think much of it til i saw her trying to eat one day, and she was turing in a tiny circle, always the the left. so i take her out of the cage and she just spins in a circle to the left.