Am I overreacting?

At work, I am doing things for a shipper that she is supposed to to. Like fixing inventory and all kinds of other things I'm just supposed to pull parts for orders.. that's it.. no inventory or anything else. She gets mad if I accidentally mess up on it even though it's her job. And today she kept bugging and bugging me saying well why didn't you fix this after you did this not before and blah blah. .. so I said okay you know what? I'm not even supposed to be doing this, honestly if you think it isn't being done right then do it yourself so when it comes back bad next time you won't have anyone but yourself to blame. And I have stomach problems sometimes when I get stressed I'll get upset stomach. And she will stick her head in the bathroom door to see if she can spot my shoes under the stall and when I don't come out immediately after about three minutes she will come back and say are you gonna come out? Are you okay? You aren't gonna go home are you?? I JUST WANT TO FUCKING SCREAM RIGHT NOW.... :(