I've got issues with knots in my back and shoulders. My doctor and PT therapist believe they are directly related to my Fibro. I do have a small cyst under my skin on the shin on my left leg, but according to my doctor it's nothing to be worried about.
I know with Fibro our muscles are really tighten up and get knots in them. Including around the ribs, I have RA/Lupus and Fibro. I know that you've been DX'd with Fibro and it's possible that your knot is Fibro related.
In the 26 years that I've have Lupus they have never said that my knots are related to my Lupus.
I hope that you can get a DX soon, I know that you believe lupus and the Dr's say no. It's a tough spot to be in, it took 13 years before my doctors agreed that I had lupus. You just have to keep fighting and seeing doctor after doctor I'm afraid.
If you're not keeping a journal of symptoms, I'd recommend you start one, if you're having rashes or anything that can be photographed take pictures to show the Rheumy. Lots of times I would have a rash or bruise or spot and when I'd see the Dr it would be gone. We now take a photo of EVERYTHING that looks not the norm for me.
Wishing you the best and hang in there.
DX- RA, Lupus, Fibro, LGL Leukemia, Pulmonary Hypertension, Bells Palsy, Gastroparsis, Blood Clots, Glaucoma, Chronic Pericarditis & Pleurisy, Severe Anemia. Way to many medications to list.