Can you explain if it's usual to feel so poorly with a tooth abscess?

Perhaps: We can't properly evaluate you over the internet nor determine why you feel so bad. It may or may not be related to your dental problem. See your dentist or physician as necessary.

Yes, it is: The infection is taxing your immune system, and you're feeling run down. I hope that you are seeing a dentist for this problem and not just letting it fester.