Mumsnet hasn't checked the qualifications of anyone posting here. If you have medical concerns, please seek medical attention; if you think your problem could be acute, do so immediately. Even qualified doctors can't diagnose over the internet, so do bear that in mind when seeking or giving advice.
I used to get this when I worked as a nanny and when had my own DSs. I put it down to how my thumb was stretched when supporting them on my hip, I'd hold their leg with my hand and my thumb would be at a right angle to my ahnd and at full stetch, if that makes sense