Can I please have some perspective on this situation and how you would all deal with it?
16yo DD recently finished school. We’ve always had a good relationship, never really fallen out. We’ve always spent a lot of time together and she’s a real home girl, she barely goes out and just enjoys staying home. She’ll go to the cinema every now and then with her friends but is normally more happy to stay in. She’s really popular and one of the “cool kids” (sorry that’s an awful term but just trying to give you an idea of what she’s like).
I never look at her phone anymore, I haven’t since she’s been 16 but I do pay the bill.
She’s always FaceTiming and I tell her from about 10pm to be quiet (small house, younger siblings asleep).
Anyway, yesterday we were going out, she wasn’t getting ready so I went into her room (knocked first) and she was on FT, told her to get off it and get ready. She threw her phone under her pillow and said it was just her friend (I didn’t even ask). I told her to give me her phone as she was acting so suspicious, but she got really agitated. I took the phone and told her to get ready. I had a quick look through her phone and she was on call to a boy who appears to be her boyfriend. No biggy but disappointed she didn’t tell me about him. I then noticed photos where she’s in her room with him, and his room too. I’ll spare you the details but they weren’t playing chess.
She started shouting at me about privacy and for the first time ever was getting angry and shoving me to get the phone.
We were getting late to go out so I gave her the phone and said we’ll talk tomorrow.
I’m upset that she lied to me and went behind my back, and that she brought someone into the house without telling me. She’s also not on the pill.
My perception of this is totally skewed as I had her when I was 17, and I do not want history repeating itself.
I’m actually gutted, I’ve always prided myself on our relationship and how well we get on and I feel like she’s thrown it in my face a bit 
I won’t over react with her, I’ll stay on a level ground but I want her to know it’s not ok to go behind my back and to bring people into the house, and lie to me.
Any words of wisdom?