Am i being taken advantage of or is this fair.?

I work for a family member (by marriage not blood) and i babysit his 9 year old daughter. I dont work on a certain schedule. Mon-thurs i work 4pm - midnight and some days i work 8am-2pm and then in the afternoon. He didnt have me scheduled to work memorial day and then called me 3 hours before he needed me and asked if i could come in i worked all day tuesday and then wed an thurs afternoon. I got $50 for that week. The following week i worked the same days same hours and only got $40 ive only worked those two weeks so far but he also doesnt leave money for any errands he needs. He also has me buy stuff he wants me to cook or if she wants to do something i have to buy it. Prior to me starting here he had almost no interaction with me and he dodged questions ihad. So there was no agreed upon rate. Is this fair or am i being taken advantage of.? What should i do.? Also this is my job for the summer and i am 17 weeks pregnant. Help please.