Need help with girlfriend

For the past 2 months, Ive had a feeling that something was wrong in my relationship of nearly 3 years. Theres a feeling in my gut that Ive so very wanted to ignore. I could not. I went out online and searched various "Is she cheating on me" websites to find answers. Some advice, I took with a grain of salt. Other advice was spot on as far as telling Qs on how she may be acting. The bottom line is I did not want to hurt our relationship based on gut feelings. We had moved in together. Im fully supporting her and her children. Yes there are children involved. I have 3 of my own from a previous marriage that I have custody of. Let me tell you, this woman I call my girlfriend I love dearly but I will let her go If I have to. Eventually, I say something to her about how shes been acting. Bad move. I had nothing but little things she has done out of character. There is one big thing that bothers me weve talked about. The overnight stays at her fathers house. Well. Thats where she says she is. I dont know for sure. One time I told her I looked at her location through a feature iPhones have and she went ballistic. "Okay so you think thats an invasion of privacy. Do you realize who I am.?" I let it go and she now uses her own Apple iD. I want to trust her. No one wants a relationship that you constantly worrying if the other is cheating. Well...She knows the overnight stays bother me but I let it go. We had an argument over it and I cut it short saying.."Im fine. You dont have to check in. Ill see about getting help for my insecurities." Yes I said this. She's extremely intelligent and well spoken, meticulous and a bag of chips. I hold two college degrees and she has none but she totally kicks my ass and makes me feel dumb at times. She had been planning to go to a movie marathon with her friend. This was an all day event. Sure thats fine but not 3 days before the event she ask. "How do you feel about me spending that night at my dads? The movies will let out late and I dont want to drive home." First of all why is she asking me when she knows how I feel about that. Well here we go. I wasnt sure where she would be so during a session of steamy love making, I cover her neck with hickees. Yes....Just let that sink in. I had given her these before but this time she kinda stiffened her neck to stop me but didnt put up too much resistance, nor did I persist in tying too much. After our love making if thats what you want to call it she wakes up, looks in the mirror and says.."Oh my God!!" "What?" I reply. "Is something wrong?" "No" she replies. Mind you she is holing back tears. I ask again and get nothing from her. Turns out she had been planning this for a long time. Needless to say shes on the movie outing as I type this but Shes told me she wont be spending the night. Why the change? I have yet to ask her. I felt evil. Like I used intimacy to she if she was cheating. If she isnt then I have some issues. If she is, I nor my kids deserve someone like this. They've already have had enough from their mom. When I talk to her. What should I do? What should I say. Im I wrong for doing what I did? Should I begin taking the steps to separate? Any help would be appreciated.