I recently leveled up to lvl 20. When I was at lvl 19, my Health points were 1025, and Mana was 96. Now, at lvl 20, my Health is 997, Mana is 123.
Shouldn't my max Health points have gone up not down?
Did you change any gear at that time? Sometimes depending on the gear you had before, and you find something, that satisfies your taste. Your new gear might have lowered your HP, and Mana.
If your mana went up from 96 to 123, you changed gear: a ring and/or athame. That and whatever other gear change you made would explain why health went down.