Hands down, I still like the Spoon Theory, when trying to explain how you feel. Christine Miserandino's The Spoon Theory This is a useful tool for many, as it can apply to nearly all "invisible" diseases: Fibromyalgia, Rheumatoid Arthritis, Psoriatic Arthritis, Chronic Depression, Lupus, Cancer, etc. The list is longer than my hands will let me type.
I have a bumper sticker on my car from Zazzle.com, but cannot seem to find it . . . here's one, though Autoimmune Arthritis Awareness bumper sticker from the IAA Movement
I do have a handicapped placard, for use at school and if necessary while out and about. I have gotten some pretty nasty looks from others, when I park there. Keep in mind, if there is a non-handicapped spot close by, I will ALWAYS park there. So as to leave the handicapped spot available for someone who may not be as lucky to find a close spot when they need it. My Bumper Sticker says, If you want to know why I am parking here, ASK. I am not lazy, I have autoimmune arthritis. Disability does not have an age requirement. On good days, I park as far away from the door as possible; taking full advantage of my body allowing me to move without pain (or with little pain, since I am rarely pain free).
Zazzle also has some amusing shirts (note, I am not affiliated with any of these sites financially; I just find them helpful). My favorite is "Today my immune system attacked my own body. What did yours do?"
It is really hard to explain to someone who really doesn't know anything about autoimmune conditions, how bad things can really be.
The fact that you're exhausted, which is due to the overwhelming percentage of cytokines going through our blood stream (others feel this when they get the flu or a bad cold with a fever; it is a hallmark of the autoimmune response), without much activity. Your low grade fever. My joints rarely swell, so it is really hard to tell outwardly that I feel like crap.
Couple this with not having a true diagnosis, or still waiting on a diagnosis, and you may find very few people who understand. Some may not even believe you (I hope you never have to go through this).
There are so many support groups out there . . . I hope you find one that keep you up, when you are at the bottom . . . Have a great weekend.