Well I have a feeling I won't be riding today - my back is in spasms :P
I will be checking her back again (I checked it yesterday, it was fine) and will be riding in a saddle next time I get on; we'll see what happens.
I mis-worded what I said in Equina's quote - I apologise, I didn't realize that's what I had said; I meant to say that when I put my hands on her neck and pushed myself back (if I got too far forward; she's built slightly downhill) or grabbed mane to balance at all (she's built like a barrel ;) ) that's when she would rear.
Really, it seemed more out of fear - which makes me wonder if there was something in her past?
Anyways, we'll see how she works under saddle and go from there - maybe she's just not a bareback horse ;)