Hi Rosie
I have had this - painful isnt it! I had the sialogram and it is fine so dont worry. It is a bit uncomfortable when they push the dye into the salivary gland as they fill it as full as possible but it didnt hurt, just discomfort. Did they tell you to "milk" the gland? Just rubbing really in one direction along the duct from the ear down to the mouth, pushes saliva and possibly the stone out.
Good luck and hope it goes ok on Wednesday.
H2x