My Son Is 1yr 8m And Yesterday When We Where

My son is 1yr 8m and yesterday when we where walking his legs just have out. I picked him up, he took a couple of steps and his legs gave out again. He then started crawling. I stopped him and checked for pain which there was none. He then tried to walk again and the same thing happened. A couple hours later he could walk but was really wobly. This morning he could walk but still a little wobly and has fallen twice. What could this be?