I got itchy painful rash bumps on pubic area after shaving?

Ingrown hairs. Those are ingrown hairs due to shaving too close to the skin. Modern razors are so good, they actually shave *below* the skin surface, which sets up ingrown hairs. Use a single-blade razor or electric razor and shave with the grain, not against it. You won't get as close a shave, but that's the idea.