Is it normal to feel this way after starting your first job?

Previous to this summer, I had a twice a week for a month little summer job teaching preschool. This summer, since i'm starting college in the fall, I decided I should try and get a more permanent part-time job that I could possibly keep during the school year. i got hired on last week at a department store. Everyone is very nice and the work isn't absolutely terrible (just the normal customer service woes). My first week I was already working twenty hours, and the next few weeks I am scheduled for around 25 hours. I'm kinda freaking out and feeling overwhelmed. Every night so far i've come home with terribly aching feet from standing all day and I've only had to work 5 hours a day maximum so far. Next week i'm scheduled two 8 hour days right in a row. I am also already paranoid about when school starts how I will be able to handle things if its already this bad. Am I just being a baby because it's my first job and I'm used to getting to sit around all day as a teenager, or do you think I'm probably legitimately being a bit overworked for what I can handle? I'm really at a loss as to whether talk to my boss about getting less hours ( even though i've only been there two weeks), say to heck with it because I don't really a need a job THAT bad, or suck it up and stop being a baby.