What Causes Left Sided Flank Pain In A Child?

For the last two days, my 7 year old son has had a sharp pain on his lower left side and toward his back when he pees and for about 5 minutes later. It s happened only at home; once last night, once this afternoon. He started school about 3 weeks ago. He drinks a lot of water at home. At school, more milk than water. No pain at school when he s had to pee. I ve encouraged more water drinking during the school day. Should I monitor or take him in to see his doctor?