How do I tell my I'm sorry after going out with my friends and Having some chick call her cell phone?

A Few nights ago I left my girlfriends house to go to work. After work I went out with some friends to a party. My girlfriend always wants me to come over her house after work and not hang with my friends to have some fun. I understand her wanting to spend a lot of time with me and careing about where I am but I don't see why she gets so upset when I am out with my friends every once in a while. Now, I did not call her when I found out I was going out and mabe thats the case. I also stayed out for the rest of the night because when I stayed out once before she said she wished I hadof stayed gone for the rest of the night than to come in early in the morning. So I stayed gone. Was I in the rong and if so How would I make it up to her. Oh, 1more thing. she found out what I was doing when a friend of mine called her phone early the next morning it was a girl I new from back home. I ran into her at a stripclub, and mistakenly gave her my girls number and not mine.