The problem is word = word + s[i];.

The compiler interpret it as word.operator=(word.operator+(s[i]));, and both string::operator+ and string::operator= is O(n). So the loop

while(s[i]!=',') {
    word = word+s[i];
    ++i;
}

is O(n²). You can generate a very long comment "ccccccc...c,0" and test your program with it. To fix this use word += s[i]. It's amortized time complexity is O(1).

In C++, a += b is not exactly same as a = a + b. Actually they can be totally different.