I Have 15-17yr Old Chi Mix Dog. He Walked And

I have 15-17yr old chi mix dog. He walked and did fine this am. When he got up a little bit ago he is dragging is left hind quarter. I thought his leg was just asleep, but it's not it has to be something else he does not appear to be in pain, I have rubbed it and checked for anything not normal found nothing. He is not wrimping when I hold him or pushed on it, and he does push back. His gums are pink and he has been drinking some water. A year ago vet said he does have arthritis so should I just watch him for 24hrs or do I need to get him to a vet asap? Never mind taking to vet she is waiting for me to get there