Don't go to HR, they r no friend of yours.
Once u ask them for the FMLA papers, you'll notice a difference in how u r treated. If it's a short leave for a family member, they will be ok. Once u let them know u r ill, from my experience, u can count on them making your life even more difficult, in hopes u just quit.
If u know u can no longer work, quietly gather all info on company policy, and any disability insurance info u have thru the company. I don't want to scare u, but I worked for big 3, and I wish someone had told me something. I would have done things differently.

I was working reduced hours and got cut off from LTD Insurance company and have been fighting for 2 years to get reinstated. June 2017 I lost my 22 years employment. Company made sure I was placed on a lighter duty job with a group of 5 and we all got let go “excuse closed department “ even though we made it a success.
My suggest is if you have insurance put a claim and do Not try reduced hours program. Of course now I’m living out of my term package with a 13 year old. Sad that we pay into it all our working life’s and I the end get the extra stree to proof you are actually unable to function as normal.
Good luck .
Ricki

My work was very understanding about my condition and had no problem making changes so I could still work. Not all companies are inconsiderate to their employees.

I had a job as manager in retail with a 43 hour contract, usually doing 50+. I told my employers when I was diagnosed, they did seem understanding at first. When work was getting too much for me I spoke to HR and store manager on several occasions about reducing hours or stepping down, they didn't want to listen. I decided to leave and get another job, I now do support work on 30 hours. I would talk to your employers and see if there is any way you can reduce your hours or days? Also maybe give them some fact sheets about fibro and see if they can try understand what you are going through. Good luck x

If this was me I would take a leaflet that explains about the illness so they can read up about the simtoms when u explain your problem. You can print off information aswell