Im 18 how do i deal with my out of control mom?

my mom and dad are seperated, so i split time between there and my dads. generally knowing my moms overprotective i save the hanging out with my friends untill im at his house. Well im 18 now and on spring break so i decided to go out with my friends last night. I went to my schools turf which she said was ok. then i went back to my dads with some friends. i kicked everyone out at 1130 and my dad was home. i tried calling my mom to tell her i was staying over at my dads and she didnt answer so i texted he :Safe at dads going to sleep. well toda my moms furious. she took my shoes! she took my phone, and when i tried to explain to her im 18 now and in 4 months im going to college out of state so she needs to adjust, she hit me! how do i show her its innapropriate to ground an 18 year old from his dads house, when i have a 3.5 gpa, i have a job, and have never been in trouble with the police? HELP