Rant about unpaid labor

So yesterday I had a little incident at my internship. I ended up forgetting my lunch at home as a result of being in a huge rush to make it in at a reasonable hour. I was hungry and grumpy and just wanted to go home to get the lunch that I had carefully packed the night before. I had asked my supervisor if I could go get it and she had said it was fine but I wanted to finish the project I was working on before I left. When I finally was leaving out the door, a woman who works in the office as well asked me where I was going and seemed annoyed that I hadn't told her that I was leaving. It wasn't even about the fact that she asked where I was going that bothered me, but her condescending tone that made me mad. I don't think she meant to sound the way she came off but it still affected me negatively and when I got into my car, I was fuming with frustration and anger.

This small incident brought me to think about my role as an intern at the organization. Now, it's different if you get paid but I do not get paid and I think that's pretty common for internships. I am treated well at my internship and am given real work that is useful to the organization but yet, sometimes I still feel a bit strange in terms of what exactly my role is. Should I have alerted everyone in the office before I left to get my lunch instead of just my supervisor? I can guarantee that if no one had seen me leave it would not have mattered since the only person I am responsible for checking in with is my supervisor. Why did it matter so much that I was leaving for the lunch break that I am supposed to be given anyways and I didn't tell an employee of the organization whom I do not interact with on a regular basis? Her reaction made me feel like she felt entitled to yell at me because I'm an intern even though I wasn't doing anything wrong. Also, everyone in the office is new. We even have an employee who started yesterday so naturally I know more about what's going on at the organization than she does. Therefore, right now, I am more valuable than her in the office but I do not get the same type of respect and compensation that she does. Don't get me wrong, I really like both the woman who acted annoyed and the new employee but this issue has nothing to do with personalities and all to do with the fact that I am unpaid, valuable labor.

Now, I know this is what I signed up for. I knew the internship was unpaid and I knew that my title as intern would mean that I am more disposable than the actual employees. Now that I'm done ranting, I think what I learned is that I need to take away everything I possibly can from this experience in terms of skills and make a good impression on the people who work here whether it is my way of doing things or not. This internship is just a small stepping stone to my future and I should avoid getting worked up about the unpaid part.