What an thoughtful and loving daughter you are! Thanks for asking us for advice and we are only to glad to help.
I'm sure your mother has explained what fibromyalgia is like as an illness. It's not an nice one to have either and doing even an simple task are hard to do on an bad day. With fibromyalgia, we never know from one hour to the next what we are going to feel like. Our pain could be worse or about the same but we may be so tired that we need to lay and rest for an while.
This being said, I'm pretty sure your mother would be glad for any help around the house you offer to do. When she getting meals ready, try joining in and making it an special time for you both. It can be most rewarding in so many ways. If you are going out with friends to an shopping mall, ask her if she's needs anything since you will be near stores. Then you can always ask her if you can do something for her.
I hope this helps an little and know your mother will appreciate it so much.
Chy