Discouraged is an understatement! (CNA)

Sorry it's long but please read and forgive me for any errors!

First of all let me say I hope everybody's day has been better than mine. I'm in desperate need of advice! I recently started my first nursing home job as a CNA (about a month ago), and it's been horrible. I'm full-time, and I'm taking one of my nursing courses to lighten my Fall semester load...so it's been overwhelming. When I first started working at the nursing home, I got really sick with a 103 temp so I had to call off for 2 days (I had a doc note). THEN I strained my back assisted a patient in standing (she was limited 1 person assist but pretty overweight). It was my first time working with her, and since that's what her care card said I went with it. I filled an incident report and of course the nurse gave me the worse attitude ever during the process. Today was my first day back at work since my back injury. My job sent me to a doctor that put me on "light duty," and switch my shift to 6am-2pm until my doc can clear me. When I got to work I was told to clip/clean the finger nails of residents, clean and organize their closets, and assist in breakfast and lunch. As far as my direct assignment goes I had cleaned 8 closets and done manicures for 8 residents by the end of my shift. Around 12 I asked the LPN if I could go to lunch and she said no because they were about to pass trays (This was my first 6-2 shift and I didn't know lunch was at 12)... So I said that I would go afterwards and help with lunch, and she said, "If you even have time to take one." Anyway, trays didn't arrive until 12:45 ( which I could've been back from lunch by then), and we didn't finish until about 1:30. When 1:30 came, I said that I was about to go on lunch. She said, "well are you working OT?" I said, "No, I'm on light duty." She said, "Well you're about to be done with your shift so it's no point of you leaving." I said, "Well HR told me to try to always get my lunch in since it AUTOMATICALLY comes out of our check." So she proceeded to ask me to take vitals for her, and inferred that she wanted me to take a 15 minute break instead of lunch. I told her no I would't do vitals at that moment, and that I didn't mind staying after 2 to do them AFTER I take my lunch because I was starving. She told me to go tell the DON that I was taking my lunch, and the rude DON cut me off when I was explaining to her that I wanted to go to lunch and told me that they schedule lunches because workers try to "do what I just did" by waiting until the end of my shift so I can leave early. She repeatedly cut me off, and said that she wanted to do rounds with me before I left for lunch. I did rounds and told her everything I did and she said that she didn't think that was 7.5 hours worth of pay, and that it doesn't take that long to clean closets, feed residents, and do nail care. I told her that breakfast and lunch is very time consuming, and it does take a while ESPECIALLY when the kitchen sends trays up late. She then proceeded to tell me not to try to take lunch that late again, and when I tried to explain to her once again that it wasn't intentional she cut me off and told me to take my lunch "before I not receive one." I walked out of her office livid at the disrespect and unprofessionalism of that place. They are obviously trying to make me quit, should I resign? The way they treating me about taking a lunch, is it legally okay? I was saving up for a new reliable car for clinicals and I can't do that if I quit this job. I honestly feel like I'm beginning to not like CNA work because of this job. I feel like I'm damned if I do what they ask, and I'm damned if I don't. If the DON didn't think that was enough work for 8 hours she should have told my supervisor to schedule me more things to do. I feel like I'm in a place full of old wolves who want to demolish an little timid kitten like me. Also, the other CNA's were making comments about race today and who would be "house slaves" and who would be in the field (ALL of my immediate coworkers are black, and I'm like the only mixed one there. They made comments like, "She would be the person that the master sleeps with!" and blah blah blah. I'm fed up, but I want to push through. I don't want to be ran away...but how do I know if it's time to walk away?