malloc and free: pointer being freed was not allocated

I've been trying to be more "memory-aware" in my C programming when I found out about the malloc method and similar memory management methods. However, when I tried to use realloc to allocate as little memory as possible to a char* instance in my program, I found that it wasn't holding as much data as I thought it would.

Here is my code:

int foo(char * arg){
    if(condition){
        //code here
    }else if(other_condition){
        char * cmd = malloc(5);
        fgets(cmd,sizeof(cmd),stdin);
        //more code here
        if(another_condition){
            //more code
            cmd = realloc(cmd,new_size) //once in this if block, but also once in another block
            fgets(cmd,sizeof(cmd),stdin);
            //more code
        }
        //more else-if blocks here
        free(cmd)
    }
    //more else-if blocks here
}

To be specific, in the above code snippet, new_size was 255, although it has been set to other sizes in other places. The problem is that when I run the program I only get 7 letters of my input.

Example output:

...
Enter filename: document1
Loading file "documen"
Load failed
...
(here the next time fgets is called it fails because "t1" is not valid input for the program)

I understand that it's receiving "t1" because I'm not clearing the input buffer, but what I want to solve is the fact that I'm only receiving the first 7 characters of the input. If I call sizeof(cmd) in the middle, it tells me that the memory occupied by cmd is 8. I also tried allocating the memory using char * cmd = malloc(5 * sizeof(char)) and cmd = realloc(cmd,255 * sizeof(char)) but it didn't solve the problem. I should probably mention that if I declare the variable using the char cmd[255] syntax and I don't call malloc, realloc, or free anywhere this problem no longer comes up.