Question about Dungeon Defenders

I think it's normal, not sure though. I don't play it much :P

Monk auras lose health just by existing, the other defenses should stay at full health.

if you're playing as SERIES EV then your defences naturally degrade as it's being used.

What class were you playing? If you were playing the monk, the Series EV, or the Huntress, this happens normally anyhow, due to the fact that their trap life is directly linked to whenever it does damage. Also, keep in mind that enemies will attack towers, so perhaps you were just not seeing the towers being hit. Otherwise, no, that is odd, and shouldn't be happening.

if u picked up any armor that adds health it will decrease

What class are you using?