Samsung SPH-M610

So I bought this phone used off of craigslist. Everything is fine on it, except i can only have a max of 50 text messages before it starts sending me a error message saying "Memory is full, message rejected". Does anyone else have this problem? I have gone through the menus meticulously searching for the reason, and cannot find anything. Phone memory says its 45mb, i have a 64mb card in there with nothing on it. The messages i send and receive are nothing more then text, no icons or images...so i just dont get it. Does anyone else have this problem, and if so, how is it fixed?