Interference/Obstruction

[sp279]

Hi all,
First Q&A for 2015!

USSSA 16U-
R on second. Ball is hit about 5 feet directly behind 2nd base. No fielder really has a play on it, except maybe the center fielder who is charging in. Ball drops about a foot or so to the ground on the grass. I am in the C position. R at second realizes the ball won't be caught (it was a arcing trajectory) so she heads to Third. In the meantime, the shortstop starts heading towards where the ball is going to land but in my opinion would never had made the play (to catch it or even field it when it landed). While doing so the R, who tried to avoid contact runs into the shortstop, knocking the shortstop down; but also the contact impedes her running progress to where the center fielder after picking up the ball could have made a play at third on the runner. I was right there and called OBSTRUCTION on the shortstop. There was a runner being forced from first to second by the hit as well. By the time the Center fielder picked up the ball the runner was just about at second and she couldn't make a play on her; but because the R on second was slowed by the contact she could have made a play by throwing to third, but for whatever reason elected not to.

The head coach charged on the field demanding that the R going to third should be called OUT for INTERFERENCE on his shortstop (who again in my opinion did not have a chance to either catch or field the ball even without the contact). The play ended with the R safely getting to third, thereby cancelling the OBSTRUCTION sign.
I would like to have some opinion in what you would have called. My interpretation is to call an INTERFERENCE on the R the shortstop would have had to of had a legitimate chance at making a play. In this case, the contact actually could have caused the R to be thrown out at third. So was OBSTRUCTION the right call?
Thank you!