Cats

I don't know how old your son is but sometimes this happens especially to adolescents for apparently no reason. Girls have more of a reason than guys but sometimes it just happens. I remember one day when I was growing up, my Mom was getting dinner and one of my brothers was in the kitchen with her. For no reason that she knows of he just slumped over on the floor. My parents helped him to the bed and had him lie down for awhile. My younger brother having observed all this was standing looking over him and all of the sudden he went over. We were all very stunned and worried for awhile. My parents just kept watch over them and it never happened again.
It could be just a fluke thing. The fact that he had just bent over to get a pen could have something to do with it, he could have stood up too fast. I think you are wise to get him a physical, but you may never know what caused it. It is just one of those things that can scare a Mom to death and cause her to think of 101 reasons this could be happening.
......calling on the Name of Jesus for you and your son!