Hello! God, sorry, that's horrible. Same thing happened to me three years ago and I was devastated (I have autoimmune arthritis and about three other autoimmune things I'd never heard of til they started attacking me!)
At first I was terrified because they give you so little information in the first six weeks, until you see the consultant.
If it's autoimmune they'll give you immunosupressants, which sound terrifying but actually are great (methotrexate saved my mobility.) I don't think meth will work with BF though...but talk to your consultant when you see him/her - write down a whole long list of questions you want answered.
Steroids are also good, though I was dead against them at first - they help you feel human again and able to move/do normal stuff, which is so important.
It's horrible when it happens when you're young because it's like - 'what, arthritis? Why???' - there is a feeling of injustice
It will improve with treatment though, and slowly you will return to having energy and being able to do things you could before you got ill.
feel free to message me.xx