Derek,
Sorry for the toil you are going through. I am in your corner and have been including you in my prayers.
As for the fluid filled lump under your tongue...it is, in my opinion, most likely a "mucocele". Any time surgery is performed in the mouth and normal or abnormal tissue cut there is a chance that a minor salivery gland gets cut. If the opening gets covered during healing with tissue a fluid filled "cyst like" sac of fluid appears. They can change in size, be painful or not. It is important to have it looked after as they do not go away on their own.
I'm glad to hear your are doing better.
If I can be of any help just let me know.

I hope you and your family have a great Christmas!!

Cheers,

Mike