Mom is mad about the day date i had

Ok so I'm 18, go to college, and work. I haven't been able to see my bf as much as i used to bc of our schedules. We were both off all day so we decided to have a day date and go out to eat and then visit the museum. I let my mom know where we were going since i still live with my parents. She was ok with it at first but as time passed by she just kept getting upset about me not being home. As we were leaving the museum, she texted me saying that she was going to have a serious talk with me when she gets out of work and she didnt want to tell me what it was about. This is not the first time she has gotten upset about me going out with my bf and i don't understand why.