I have to tell my mother something and i'm nervous about it?

a month or so ago, my mom and i were talking about putting me on her insurance through work. i'm 22. i asked her to find out about it, how much it costs and so on. at the end of oct. she called me and told me i was on her plan, and that i must pay her $150. first off, i was kinda mad that she put me on before i had a chance to hear the details and agree. $150 isn't exactly cheap. i became unemployed a few weeks ago and i have not told her that i lost my job and that i cannot pay her the $150. i feel bad about it but on the other hand i don't. i know we made an agreement but our family standings have not been the best, we don't talk much and she allowed my 30yr old sister and her boyfriend to move things out of my room and take my room. i had been staying with my bf because of the toxic living environment i was in at my mother's. in addition, my mother never gave me a birthday present. i know she could have afforded one because they had just gone on vacation. i know she can afford to pay the $150 until i can pay her back, but i know she will flip and her husband will give her crap about how i'm a bad daughter. what can i do?