Just wondering if you ladies have any suggestions why I might be so tired all of the time?

I wake up exhausted no matter how much sleep I get (some nights it's 2 hours, some nights it's 9, or anything in between), and feel tired all day. I keep busy (SAHM running a business from home) and find that if I do sit down to relax for 5 minutes, I struggle to stay awake and struggle even more to start work again or generally get back up. I usually hit a wall and get extremely tired at about 9pm (usually awake at around 7am) but I'll start to feel more alive again at around 10pm and I can stay awake then for another couple of hours. Baby usually goes to bed at roughly 8pm so once he's down I'll do some paperwork until I hit that wall, and then that's me time from then on.

I've tried working less, getting some help with the baby during the day, taking full days off to just relax, and no matter what I'm just exhausted. Even on holidays when I've had a few days off in a row.

I'm apparently not anaemic and have a thyroid that works properly (though I was VERY low on iron throughout my pregnancy and was monitored for my thyroid so I'm not sure I'm totally in the clear there) and my doctor isn't sure what else it could be.

Any ideas?