After a spider attack etc. it says you were poisoned. What exactly does the poison do? I don't notice anything...

go into magic, then active effects i think then scroll down to you see a red word it will tell you what the posion is doing to your character. Travel to a temple (eg the one in whiterun) to get healed. Hoped this helped

It usually just drains health for a couple of seconds and your screen gets kind of hazy around the edges, if I remember correctly.