Correctable error question

[jTheUmp]

This happened to me Friday afternoon.

9th grade girls game, 2-whistle mechanics. A1 is fouled on a layup attempt. Shot does not go in. I make the foul call as the lead. I report the foul, take my position as the new trail. My partner mistakenly thinks the shot went it, says "one shot" to A1. A1 misses the shot, B grabs the rebound and starts a fast-break down the court.

All the while, I'm having a brain freeze. About 3 seconds later, B is about to attempt a layup, when suddenly the "OH #$@*, there should've been two shots" alarm goes off in my head. I pause for a second or two, B attempts the layup and misses, A rebounds, and I blow my whistle, stop play, and return A1 to the line to shoot her second free throw.

I know it got it right on the "correctable error" and within the proper time frame. But I'm not sure if I should've stopped play when I did.

So, when should I have actually stopped play?
a) as soon as I realized that we didn't award the second FT attempt? (Prior to B's layup attempt).
b) as soon as A secured possession after B's missed layup? (what I actually did)
c) wait for the next dead ball (successful try by either A or B, or violation or foul by either team)?
d) other (please specify)