If this is your first job, welcome to the working world! You are going to have many, many years of dealing with all kinds of bosses, so you need to realize how things work.
Restaurant work is demanding, tiring, and unrelenting. We live in a me-me-me world, so pleasing everyone is a struggle.
If you suffer from frequent nosebleeds, you are going to have to get this problem under control if you want to keep this job. Your absence from the floor creates a disruption because someone has to step in and do your job until you return. So someone is pulled off what they are doing to cover you, or this is added to their workload, so they are doing their job and yours.
If you keep leaving, this becomes quite the problem. I can understand why your boss would be impatient with you if this is an ongoing issue.
I'm not sure what your state's requirements are, but here if you work 6 hours, you get a 30 minute unpaid lunch break. Employers are NOT required to feed you - that's your responsibility. They just need to give you the lunch break. As far as being rested, that also is your responsibility.
If you are being worked too many hours, cut your availability or change jobs.
