OpenGL pitching problem

I've been trying to implement several camera movements for my application. So far yawing, rolling, strafing, walking has been working properly, but I can't get my pitching to work properly. If I continue pitching upwards, it doesn't rotate back 360 degree, and instead gets stuck looking at the top of the screen.

Here's my code for Camera class

class Camera
{
    float m[16];
    Vector dirForward, dirUp, dirRight;

    void loadCameraDirections()
    {
        glGetFloatv(GL_MODELVIEW_MATRIX, m);
        dirForward = Vector(m[2], m[6], m[10]).unitVector();
        dirUp = Vector(m[1], m[5], m[9]).unitVector();
        dirRight = Vector(m[0], m[4], m[8]).unitVector();
    }

public:
    Vector position;
    Vector lookAt;
    Vector up;

    Camera()
    {
        position = Vector(300, 0, 100);
        lookAt = Vector(0, 0, 100);
        up = Vector(0, 0, 1);
    }

    void rotatePitch(double rotationAngle)
    {
        glPushMatrix(); {
            glRotatef(rotationAngle, dirRight.x, dirRight.y, dirRight.z);
            loadCameraDirections();
        } glPopMatrix();

        lookAt = position.repositionBy(dirForward.reverseVector());
    }
}cam;

I call the gluLookAt funtion by-

gluLookAt(cam.position.x, cam.position.y, cam.position.z, cam.lookAt.x, cam.lookAt.y, cam.lookAt.z, cam.up.x, cam.up.y, cam.up.z);