Not sure how much I can take

I got full time little over a month ago with a new shift. They told me it was going to be hard, but this isn't anything I can't handle. I have had bad days, impossible days, but I got it done. My body, however, is not handling the stress very well, even on good days. Going to work my nerves tense up.  During work my gut and back, usually my neck, betray me. I walk around worried I might pass out.  My sleep is worse then usual, which is saying something. Tonight my head hurt so bad I was seriously considering going to the ER.  I can do the damn job, I know I can do it, but why does my body insist on heeding my subconscious? I have to figure out how to relax in my confidence. I have to figure this out.