putchar ends only on pressing enter (C)

Question

So this simple program is suppose to change each occurrence of 's' or "ss" to "th".

Here is my code:

#include <stdio.h>
#include <ctype.h>    

.
.
.

printf("Enter a sentence:\t");

int ch;

while ((ch = tolower(getchar())) != '.'){

    if(ch == 's'){
        int nxtCh = tolower(getchar());
        if(nxtCh == 's'){
            putchar('t');
            putchar('h');
        }else{
            putchar('t');
            putchar('h');
            putchar(nxtCh);
        }
    }else{
        putchar(ch);
    }
}
putchar('.');

Once a period is entered, the program is still waiting for me to press enter for everything to actually register. What I want is for the program to end the while loop immediately once a period is entered.

Am I doing something wrong? How can I achieve this?

I am working through Xcode on Mac

Thank you!