Did the doctor know about you coughing up clumps of nasty stuff? Did you bring a clump in for the doctor to see, or maybe even send a fresh clump to the lab for culture for infection or something?
I think we all have a lot of respiratory clearing of junk when we come off smoking, through vaping or any other process. It usually gets better over time. If it's still happening, especially several months after you stopped vaping, it could be something else. Bringing in a sample of nasty stuff/sputum might very well be worthwhile. Give your doctor's office a call. They can probably have you pick up a sterile plastic bottle for a sample. At worst you end up with an extra plastic bottle for DIY juice mixing.