Well, again, what you've described does not sound like diabetes at all. However, being very underweight might be a red flag for undiagnosed type 1 - but you would also have lethargy, weakness, frequent urination, blurry eyesight and maybe other symptoms. So, if I were you, I would write down what has been happening and when it started, etc., also what you've been eating and doing and feeling and if you've been losing weight lately. With this info in hand, you can explain all this to the doctor whenever you go.
In the meantime, try to eat balanced meals, don't skip meals, take a multi-vitamin.
I don't know about
arthritic knees but try not to sit as long. Try getting up, standing a little, walking around a little during the day and at night, keep your leg(s) elevated when you are sitting down.
Don't think negatively! "I'm a mess!" Yeah, I'm a mess, too, if I list all my ailments, etc.
But I also think that our mental state can affect our physical state, so please try to do the best you can to take care of yourself and see your doctor when you are able.
Keep us updated! I hope you're doing better today!