how about you sit her on the toilet before she has a shower and if she doesn't do a poo say it will be ok now you have no poos in there.. and just keep it light.. ie your voice..

My DD did a poo in the shower (well numerous times now lol) she was upset because of it and didn't like it and we finally worked out it was because she thought the shower was still dirty so we got her to watch while we cleaned it..