I know that this situation for deciding on the ER or not is past (being it's already Tues.) however for future info...

Asprin/Tylenol/ibuprofin etc. are DEADLY to cats!

At 13 it's possible the cat has the beginnings of arthritis (Dusty was started on glucosamine at around 13 yrs old)

However being the cat is holding a leg up following excercize with the dogs, I'd be concerned about having pulled a muscle or injured the leg.