Behaviour different when debugging

I'm creating a TicTacToe Game, with classes, using C++ for school.

Part2 requires me to implement a single player mode (me vs the computer).

I have a matrix 3x3 and the function to randomly insert coordinates (CPU's move) always plays in the main diagonal (1,1;2,2;3,3).

If I put a breakpoint when I call the "random" method and debug step by step, it creates other coordinates, besides the main diagonal (which is good).

I don't understand why it only works if I'm debugging step by step.

Here's the random method:

void Ponto::aleatorio() {
    srand(GetTickCount());
    x = (rand() % 3)+1;

    srand(GetTickCount());
    y = (rand() % 3)+1;
}