What is up with my boss?

I work in design and normally i would be designing brochures and advertising for this company. Out of the blue he didnt want me in the office for about a month, then the week i came back i was a little out of practice and made a few mistakes for which he gave me a warning. now he doesnt trust me to do anything anymore! all i do now is tidy up and help the canvas boys. it really upsets me because i know im better than this! he makes me feel like an idiot. im also only being paid 4.60 and hour which is pennies compared to everyone else. but he says this is because im part time...is he being a jerk or am i just seeing this from my point of view?