Welcome to the forum Maenab.
I would expect that yeah probably you do have this sickness everytime you get the flu. I thought for a long time that because Lupus is overactive in its pursuit to attack bacteria, that we would never get the flu or other bugs and have them last very long.
Others will come along I think who can help you better than I can to find the answers you seek. before I came here to this forum, I tended to not pay much attention to this disease. Pain was due to lupus, so I did not pay much attention to it, because there is no cure for it. But the people here guided me to a place where I now know the importance of paying attention to the pain and sickness that Lupus, or some other autoimmune disease can cause.
I was advised by people here, and it was excellent advice, to keep a journal of the symptoms I experience through the day, every day. then take it to my doctor when I see him again.
Getting it to him timely is not working as well as it could have, but i did get them to him eventually. and I have started on my new pain journal to take to him next time.
Not only is this a good thing to do to take to the doctor, but also I can read my pain/symptom journal and know that I need to be a little less critical of myself.
Sorry for the long post. Hope you find the info you want.