Am I being taken advantage by being labeled as a private contractor ?

I was a recent graduate wanting to get working experience. I started volunteering and everything was going great. The person in charge decides to give me an offer as a private contractor. Basically they will pay me 5 hours a day and then I would volunteer the remaining 3 hours. So in total I would get paid 20 hours every two weeks and volunteer 18 hours. Since I was excited to learn new things while getting paid, I accepted. I been working there for over a year now but I feel like I’m being taken advantage of as being classified as a private contractor. I have a set schedule, I work from 9-5 M,T,F and 9-2 W,TH. I do mostly everything in the lab, I believe I am a key employee because the study depends on the patients I recruit. My tasks involve recruiting multiple patients, collecting and processing their samples, I do data entry, and other lab duties. I also do a lot of overtime work because I’m invested in the study while not being paid for it. The person in charge tells me that that’s the only thing that they can offer because the study is a project with not many funds. But I feel like I am being taken advantage because I contribute a lot to the study while not getting credit for it.