In the act of fielding
 

At what point does "in the act of fielding" end? Is F5 in the act of fielding when she is running to retrieve an overthrown ball?

I had a siatuation in a 10U game where there was a play at 3rd. The infielder overthrew F5. The ball went to the fence. The runner made it safely to 3rd and when she realized that the ball was overthrown, she took off for home. At about the same time F5 realized she needed to retrieve the ball. There was a collision. I signalled the Obstruction. Is this obstruction, or just a normal part of the game that I should have let go? There was no play at home so it didn't effect the outcome of the game.

Your sitch was obstruction.

"The act of fielding a ball" is not a phrase found in the book anywhere. Even though this girl was in the act of fielding a ball, it doesn't matter - she was without possession of the ball, in the runner's basepath, and caused (by contact in this case) the runner to slow or alter her path. Textbook OBS.

The phrase you WILL find is "in the act of fielding a BATTED ball". HUGE difference, and the one that separates your sitch (which is OBS) from a collision that is interference.