Ahhh yes..the diagnosis. Such a blessing and a curse at the same time....
I can only tell you what I would do in your shoes - and that is...tell them. Gently. You don't have to make it sound big and scary, or maybe as bad as it really is, but tell them that you have an autoimmune disease (explain what that is), called Lupus (explain what that is), and that there is no cure, but it's not really life threatening, and with some time and Dr. Help, you could get it under control, and learn to live with it.
Personally, I can't stand it when people dont tell their parents or grandparents what's really going on, just because they're old. Old people are tougher than you think, and i just think that if I got to be that old, I would hate it if people kept me out of the loop. So I would definitely tell your parents. Maybe one at a time, or together, depending on how you think things will go. If you're more concerned about your mother freaking out, then I would talk to your father first, to give him a basic understanding of it, and then talk to your mother while he is there, so he can help to calm her down and explain things to her, while he's not in as much shock himself.
When I first got sick, I kept it under raps as well. My boyfriend knew (because he saw me being sick every day), and I eventually told my mom after about 3 weeks of feeling like...um..butt, but I didn't tell anyone else. Flash forward to like 6 months later when I was diagnosed, (well, not really. Just told that I had an autoimmune disease), and I didn't know who to tell or how to tell them.
My boyfriend was there with me when I was given the news, and he didn't really say anything. I told my mother, and she was just happy that I found a doctor who believed me and wanted to help me (I'd gone through 4 docs).
I told my brother, and he... well, i'm still not sure how he's taking it. I don't think he can, really. He just went completely..silent. He'll ask me how i'm doing, and when i reply back about a lot of stuff, he'll go silent, and then kind of skip over my sickness. Sometimes he will say "that really sucks", though. It's all up to the person. Sometimes people just...don't want to know. But I personally think that it's important for people to know, at least about the fact that you have a disease, and what Lupus is, as well as what autoimmune is. If not for you, so that they have more understanding of you and can give more leeway, but at least for everyone else who is living with this disease that nobody has any idea about.
It's understandable to be angry and upset that you've been dealt this hand. Heck, if someone wasn't i'd be suspicious.
Just, try to take it one day at a time, and you'll get through it. Don't worry about not letting your hubby talk to anyone - it was what you needed at the time. Just maybe let him do it now (: I know I was really worried about what my boyfriend was telling everyone, and what they knew...it made me feel really self conscious at times. But you just have to let go at some point.
Best of luck to you - feel free to scream and let it out. If you get really angry and want to throw something - try taking a bucket of ice and throwing it into your tub. Not the bucket, but the ice, a handful at a time or so. It's cold, but it helps to get the frustration out, and to watch something shatter.
One day at a time, one breath at a time... We can get through this.