Be careful, you could have one of those rare rolling hiatus hernias. They are incredibly hard to catch on an endoscopy, I know because they did once (had a lot heartburn at the time) ,but I've had several endoscopies since then and nothing. So I never got that fixed, but I still get that odd pain in the middle of my ribs... as well as heartburn from time to time.
There is also a chance you suffer from IBS... a lot fibro suffered have that (I do too), one of the things IBS can cause is heartburn. Whatever you do don't use meds like omeaprozole, that med actually made it worse for me. Plus if you use it long term you might be calcium deficient.
Have you tried to see if sleeping in a sitting position helps? When is your heartburn worse?