Autoimmune diseases are when the body begins to attack itself mistakenly. Immunocompromised means you have a compromised immune system. So yes, if you have an autoimmune disease, you have a compromised immune system.
However, you do not have to have an autoimmune disease to have a compromised immune system.
The average weight of a 12 year old is 93lbs, so 140 is a little bit overweight, but not too bad depending on your height.