I've been getting headaches, with abdominal pain, burning eyes, loss of appetite, and very fatigued. What could it be?

Headaches +. The answer depends on more information... For example, which symptoms come first, and which follow. That would give some hints about what the primary cause might be. Also, fatigue suggests that there may be a sleep disorder underlying and/or contributing or causing this. I suggest discussing the problems with your family doctor and perhaps a sleep specialist.

Related Questions

High blood pressure, rapid weight loss, complete loss of appetite, lower back pain and swelling, headache, stomach pain, burning watery eyes?

Multiple. Multiple body organs seem to be affected. I'd suspect some sort of endocrine issue but the best way to tell is to address concern with your regular doctor and have a few blood and urine tests performed.
Impossible to dx. Not enough information to be of much help. I'd think about a kidney stone (back pain, stomach pain, loss of appetite) or a gall stone. But there are many other possibilities. Please see your health provider.