At that distance with a rock sparker and a patched round ball with open sights... I am not sure that the tilted verticle string is anything that would be of concern. What kind of rest were you shooting off of? At a 100 yards a very little difference in muzzle area is amplified even more at 100.

Normally but not always stringing and even a tilted string like that would indicate the barrel warming and pushing on a pressure point in the stock. Especially if the the hole near the bottom was the first shot and then it continued to string up and left. Probably a pressure point near the end of the stock on the right bottom side of the barrel...

If it were the opposite it might be the recoil causing the barrel to settle into the the stock...

Just guessing...