Laugh and call it a first world problem if you wish, but I too am currently experiencing pinky strain in the same manner. I'm not an inactive person either. Strain of any joint can occur when you are doing something that you normally didn't do before. I went from a 5S to the X, so the size and weight increase was significant. With having to access the control center from the top right and other different tasks associated with the X along with the size and weight difference, it's not surprising to me at all that I'm experiencing this. In fact, I googled "pinky strain iphone x", and this was the first hit, which is why I'm here. Was just curious if I was the only one and apparently I'm not.
As with any injury, it takes time. You need to be careful not to keep straining it, and let it heal. Then gradually begin to work it harder so that it builds up the strength and memory to use the phone without injury.
Repetitive strain injuries are commonplace now, and just because it's a "pinky" and it's because of using a cellphone doesn't make it funny, nor any less uncomfortable.