What should I about this situation I have at work?

I've been with this law firm for almost a year now. August 13th will be my anniversary of 1 year. I am a file clerk out of atleast 15. We each have our own department that we share with one more person. The person I worked with was a temp for 2 years until she was finally hired about 5 months after I was hired. She is nice, but she also has a side that i don't think she even knows about. She kind of orders me around in a nice way, but never asking just telling. I get her food and sometimes I do offer. I get her office supplies even if i'm not going down there. If I had a ton of filing it's like she over looks it and say that I have to help her with something pointless. The thing is she has been here longer, but she isn't in charge. Our boss made that very clear that we are on the same level. She completely disregards that and she breaks most of the rules. She has gotten better, but she is still doing some bad stuff. Now, I do go on the computer from time to time and if i have to call my doctor I use my phone. That's not the problem. Right now it's her being a neat freak and trying to involve me. She comes up with these stupid pointless jobs and expects me to just drop my work and help her. And I have been way to busy to help. She also volunteers me to help others while i'm busy. She has me call in things for her. "Jess, call in and get boxes." "Jess, look this up." "jess, go down stairs and get folders." LIke, wtf you are doing nothing! You can do it! It's all because I don't know how to say no. And now it's causing stress and i get this horrible pain in my chest and i can't breath and i want to cry because she is taking advantage of me and i don't know how to stand up for my self because i don't want her to hate me or there to be any problems in my office because I like this job. I'm not sure what to do, but one day i think i might explode on her or my self and end up getting fired. Sometimes I go in another room that is vacant to file just to get away from her. I don't know what to do!