there may well have been something scary down there that she wasn't having that you couldn't see/hear/smell/intuit.
ditto this!
i would recommend handwalking her, working with her on the ground in your be-nice halter in that same area and see how she reacts. once she's relaxed in-hand, THEN you can try it mounted. i'm a firm believer in fixing things like that from the ground (to avoid ending up on the ground! :P ).
good luck