Hello and welcome to HCM
Is the swelling around the coccyx
painful, is the swelling mobile or fixed to underlying structures?
Do you any associated tingling
or altered sensation in your lower limb?
Since, you are suffering from fibromyalgia
it could be due to fibromyalgia.
You need to get a MRI done to know the origin of the swelling.
It could be a bony swelling or soft tissue
Thanks and take care
Dr Shailja P Wahal