Wrong of me?

So yesterday I had this customer and they started telling me how to do my job. So I had asked them if they had ever worked as a checker in a grocery store before. They had said no. So I couldn't help myself and said "then please do not tell me how to do my job, I don't tell you how to do yours." I have dome this for 4 Years. I always have a customer telling me how to do my job and get irratated with me when I can't understand them when they give me their phone number to look up their rewards. Customer said nothing back to me and kept her mouth shut. If you've never done someone's job don't tell them how to do it. I honestly don't care if I get written up for this. Customers think that they have all the right in the world to treat you like shit and that you are dumb. And yesterday I was just in pain, my Braxton hicks were happening ever two minutes, and then my cervix getting kicked with my leg going numb because of some nerve she's on, and I worked a 9 hour shift with out a break that day because I got there 5 minutes late and my manager thought I didn't deserve one. So I had no will to keep my comment to myself. And for those that say that's nothing good for you. But try working as a cashier. Twisting your upper body all day with a bruised rib and all these ending pregnancy joys. And I'm sure some of you sit in a chair all day or you actually get to walk around. I don't, I stand in one spot all day long.

I do feel bad about it today. Just yesterday I didn't have the energy to hold it back. And I'm tired of customers thinking that they are better than me. In reality I'm better than them because I would have never treated someone that way.