shliv, your story sounds all too familiar to me. Our teen DS has Lyme arthritis in his knee and same symptoms as yours. It's my understanding that by the time it is causing swelling in joints it is late stage Lyme. That has been our situation which means that our DS has several co-infections that also flourished.
You have good advice here. Definitely keep the LLMD appointment. That is good your doctor is willing to allow longer antibiotics. I do know of a young child who had 2 months of IV abx and recovered quickly so far. Our DS had Lyme over a year before it was diagnosed. I started a website for parents like you and me:
www.parentoflymekids.com. It has our story and lots of links to information about
questions you probably have at this point.