Borland C++ 3.1 compiler Memory Capability

Hi,
    I am using the 3.1 complier, because of it's DOS support, and BGI
graphics. I am writing in C, as opposed to C++, prodcuing a DOS program,
with BGI to display a menu type system.

   I have written many quite large projects with it, but have just come
across a strange problem. I have just added 100 bytes for a new global
buffer. Suddenly, my file retrieval functions using fopen etc, fread,fwrite
return errors.

It's as if I have come to a memory allocation problem, but the total memory
according to the project manager window (^WP) is only 46280 bytes of memory.

Has anyone any ideas as to how to try and track down this problem.

David Frost