Hi My 5 Year Old Boy Vomitted 2 Days Ago.

Hi My 5 Year Old Boy vomitted 2 days ago. He seemed otherwise fine. Slightly less appetite. Now he complains of his lower back (sides) being sore when he poohs. It is now 2 days later he vomitted once yesterday morning, then was fine all day, but again this morning he had small vomitt slightly pink colour. He seems fine otherwise, happy. but still said his lower back sides hurt when he poohs? Is this a kidney infection.