Early November last year, I started getting very tired. My body felt run down and physically exhausted. I figured I wasn't getting enough sleep. Then I got a solid 12 hours sleep each night for 3 or 4 nights in a row and it made no difference at all; actually I was more tired. I started to feel like I was sick without actually being sick; no symptoms other than general fatigue and body aches. I thought I had mono.
I didn't have a primary doctor at the time, so I went to an urgent care facility (not an emergency room). They begrudgingly tested me for mono, saying they didn't think it was that. Turned up negative. They tried telling me I was depressed, which after I heard them say it for the 4th time I got really aggravated. I told them pretty sternly "I'm not depressed, now I'm pissed (angry in UK English

). I'm not taking depression meds until you rule out everything else it could be and there's no diagnosis left."
They didn't like that, so they did a bunch of blood work to shut me up and prove I was wrong - HIV, thyroid, Lyme disease, blood sugar, flu, etc. They took about 8 vials of blood.
It came back 3 days later that I tested positive for Lyme Disease. Fortunately for them, the doctor and nurse that initially saw me weren't there when they called to inform me. I had a lot of not so nice things to say to them. Then again, they'd have probably said my anger is a part of depression and I need meds for that too.
If you think depression can be a cause, then by all means treat it. If you're confident it's not, demand to be tested for other things. Had I listened to them and walked away, I'd be far worse off.
I'm not sure how common Lyme Disease is where you are, but it's getting too common here. And it's misdiagnosed very often. There's no definitive test for it, and doctors miss it often. They go by a combination of blood markers and according to the textbooks, I didn't have enough markers to officially be considered positive. My primary doctor explained it all to me as he's had several people with it lately so he's become better versed in it.
I needed 3 rounds of 3 weeks of antibiotics to get rid of it. I had it from early November until early March. The most physically miserable 4 months of my life. I know people who've had it longer, and didn't have it as long. I also know too many people who were misdiagnosed and now have permanent problems because of it - nervous system, joint damage, etc.
And if they tell you you don't have it because you didn't get a bullseye rash from a tic bite, tell them bollocks; less than 40% of people with it develop the rash. I'm pretty sure that number will drop as more research is done.
Just an FYI.
Lyme Disease | Lyme Disease | CDC
I can tell you more about my experience with it if you'd like. The thing is, everyone has different symptoms and experiences. More so than most other conditions.