Yeah, it is very tough. I used to let myself get into the mindset where I had to help physically way more often than I do now. These days, I usually just say I can't, and ask if there's anything else I can do...there must be other things that you could pitch in with? Running errands? - something you can do without causing yourself to flare?
I do get some weird looks...(oh well)...it's so hard for people to understand, because technically, you are physically capable of doing it. You seem to them to be moving normally while you're doing it, at least for awhile. But - while they might be a little sore for a day, you could well be in serious pain for days, not to mention wiped out, nauseous, and whatever else Fibro feels like punishing you with!
Five years ago, I was going through a horrible time. I was very close to my parents. My mom died in June. I moved my dad from an apartment to assisted living to a nursing home, where he died in October. I plowed through all the signals my body was sending me, because there was way too much to do and nobody but me to do it. I was in "what's next?" mode all day, every day. If I'd just slowed down, I'd have realized that there were people who might have been happy to help, if only I'd asked (I could have hired people to pack and move more, if nothing else.)
My father was dying from ALS (which none of us, including him, knew until after my mother died) and I was totally overwhelmed....definitely not thinking straight. In the end, I was less "there" for him because there was so little left of me.
My body finally let me have it in a big way and I was flaring so badly, I didn't even grasp it when he was saying goodbye...I made decisions I still regret.
I'm also sure that I did a lot of damage to my body, mostly by lifting. Three years later I had a ruptured disc. (Sure, these things happen anyway, but I know what I did.)
I see in your signature that you've got disc problems too. You've got to put yourself first....you're the only one living in your body, and it's the only body you've got...and if you don't protect it, no one will! Ok, I'm off my soapbox now. I really hope you feel much better than you expect to tomorrow! 
Debbie