It bothers me, please give me your opinion.

I started to work part-time in this company about one month. Orignally, we both agreed my work hour is 4 hours/day.(8:30am to 12:30noon)

I have been working 6 hours/day since I started. In the first two weeks, she would asked me if it's okay to work 6 hours, but she stopped asking last week and scheduled the 6 hours work day.

I sent her an email yesterday and said "Starting 01/18/2011, I will work from 8:30am to 1pm because I am going to take some classes in the afternoon."

She yelled at me this afternoon and said my email is inappropriate since I need my manager's approval for the hour changed. She felt like I was telling her this's my work hour and didn't ask for any approval.

But I don't see I did anything wrong since we both agreed I only work 4 hours/day when I got this offer. 

This is a very small company and I feel that they don't want to pay for a full time employee but would like to have someone work close to full time but get a part time pay....just being cheap.