It's kidney disease. And yes it can improve. I have been told hydration is very important, as well as avoiding certain meds (NSAIDS). Only use tylenol. Other issues with salt, blood pressure, potassium, protein in diet etc. etc. and recommendations are different for each of us.

If you were at all dehydrated for the first test, that would make your eGFR worse. Do you have a Cystatin C done?

Your vitamin D is very low. Is your doc dealing with that?