Severe fatigue, joint pain, weakness and flu-like symptoms for several months now. what can it be?

Many possibilities: You need to see your doctor. Symptoms that persist like this can be due to rheumatoid arhtritis, but many other conditionstoo. Your doctor will take a history, examine your joints, skin, lungs etc., and possibly order some tests to try and narrow down the possibilities, but you won't get a diagnosis on the internet. Use the internet to get the questions to ask your doctor, not the answers.