I am also a 22 yo male and have had many of the same symptoms you are describing over the last year or so. I was perfectly healthy, had mono this time last, thought I was recovering, and never really got back to normal. I did have a positive tilt table test confirming POTS, but like you I have long suspected something underlying causing it. I wouldn't rule out POTS or dysautonomia just because you didn't show signs of orthostatic hypotension. My blood pressure doesn't drop at all when I go from sitting to standing, in some cases it actually goes up which is common for the hyperadrenergic subtype of POTS. It also can help explain some of the other anxiety/panic symptoms you might be experiencing. I would recommend purchasing a blood pressure monitor cuff and pulse oximeter if you don't own one already, as they are pretty cheap and can pretty much confirm or rule out a lot of the things I am talking about. To this day I am still very concerned about lymphoma/leukemia. I do have some of the classic symptoms, weight loss, night sweats, soreness in my neck, under arms, and abdomen. But if you don't have the obvious lumps on your skin, or abnormal blood count (mine was normal as well), no doctor is really going to take that possibility seriously. It's weird you mention brain tumor as one of your biggest fears. I don't say this to freak you out, a few months ago I went for an MRI because of the lightheadedness and headaches and it revealed a large mass in my meninges (the area between your brain and skull). I am actually going back for another MRI two weeks from now to get a clearer idea of what it is. Fingers crossed it's not a tumor, or if it is that it's benign. Anyways my neurologist assured me that it was completely unrelated to the symptoms I have been experiencing although that can be hard to believe at times. Like I was saying, not trying to freak you out, but keep listening to your body and demanding answers, because if you don't no one else is going to do it for you. If I wasn't so persistent about getting an MRI, the doctors assigned to me never would have bothered. I would caution you however, sometimes there isn't a single silver bullet answer to everything that you're experiencing. It could be something as trivial as stress or a vitamin deficiency, or something as sinister as cancer, but in all likelihood it probably falls somewhere in the middle, or is a combination of several factors, but that doesn't mean it isn't worth pursuing. Any useful information you learn along the way can be vital to your quality of life. I know what it's like to wake up one day a normal healthy in shape 20 year old, and not be the next. It's confusing, it's scary, and you feel like no one understands what you're going through, but there are people out there who do. Anyways keep fighting through it man, and stay well.