STRESS SEEPS IN WITHOUT YOU KNOWING

I only work 8 hours a day. Now surely, as anyone would think, i wont have any stress since i can enjoy myself after work.

But..

I've been having work related nightmares for nearly the whole week now.
Keep dreaming of the worst case scenarios that could happen.
Now if thats not STRESS, i dont know whats that.

NOW the real qn is, why am i stressed?
i've been pondering but i cant find the answer. :(