I'm A 21 Year Old Male, And I've Been Feeling

I'm a 21 year old male, and I've been feeling bad lately. I've been very fatigued, and I have been losing weight. I've been having diarrhea a lot, and I have a very bad skin rash on my back, upper shoulders, and chest (started a few months ago). Last week I also had some lymph node swelling (the back of my neck, and front of my throat). I have been to a doctor, but he didn't what was wrong.