How to help a dog recover mentally from an injury during training?

I want to start off by saying that she's been to the vet twice since it initially happened, and only acts fearful when the clicker/training treats come out.

About four months ago my almost two year old rough collie suffered some sort on injury while training. I took her to the vet the next day and they couldn't find anything wrong, so we assumed she either got stung by something or cramped a muscle. I was luring her through a roll over when all of a sudden she just yelped and jumped back. She was scared of going on walks and spent most of the next two days in her crate avoiding us.

She used to be all over training, but ever since then any time I try to start she gets anxious and almost shuts down. She'll eaither walk away curiously or lay down and show her belly submissivly. It truly breaks my heart.

I've been trying to get back confidence in the basics like sit, down, and spin for about a month now and she's still anxious. Hell she gets anxious if I so much as sit on the floor and give her training treats for approaching me. I've changed the treats, switched to a verbal marker instead of a clicker, and started training in different locations, but nothing seems to help.

She'll take a treat happy as a clam if we're not training and is otherwise a Velcro dog, so it's so strange when she acts like I'm about to hit her when all I'm asking for is a sit even though she had been jumping through my arms five months ago.