Previous posters are right, all you can do is be prepared for your next appointment. Don't start letting your imagination run riot on 'what it might be', you have only had normal tests so far.
Questions to ask might be 'what is your differential diagnosis?' if you want to know what your doctor is considering, but bear in mind this will include the very unlikely problems as well as the likely ones.
If given a choice of options, the best question is 'if it was your wife/mother facing this, what would you recommend?
Ensure you get a copy of any referral letters so you are kept in the loop.
And write down a list of your worries. Doctors have no clairvoyant skills, so they can't reassure you or explain if they don't know what is causing you concern, which they will naturally want to do. Good luck, and give yourself some little treats and things to look forward to! 