Why are my strings changing unintentionally?

I'm making a simple caesar cipher decryption program in C. It takes a string input (stored in text) and should print every possible shift in letters.

If I were to give ABC as input, it should print:

ABC
BCD
CDE
DEF
EFG
...

However, it actually prints:

ABC
BCD
DEF
GHI
KLM
PQR
VWX
CDE
KLM
TUV
...

The code is:

int main(void)
{
    // prompt for plaintext
    string text = GetString();
    string newtext = text;
    for (int k = 0; k < 26; k++)
    {
        for (int i = 0, n = strlen(text); i < n; i++)
        {
            if (text[i] >= 'A' && text[i] <= 'Z')
            {
                newtext[i] = (text[i] - 'A' + k) % 26 + ('A');
            }
            else if (text[i] >= 'a' && text[i] <= 'z')
            {
                newtext[i] = (text[i] - 'a' + k) % 26 + ('a');
            }
        }
        printf("%s\n", newtext);
        printf("%s\n", text);
    }
    return 0;
}

I put printf("%s\n", text); only to debug, and I found that the text string is changing every iteration of the loop, instead of only the newtext string.

Also, I'm including some header files already.

Why is text changing?