Girl text me and my girlfriend has completely changed!

Hi everyone

Okay so I have been with my girlfriend for five months now - recently things have been slightly tense between us - I think it's to do with the fact that we share all the same friends and we see each other everyday, even if we are just chilling out with friends. Anyway last night I was at her house and my phone was on the sofa arm rest and it vibrated and she asked if I would cheat on her - I told her that I'd never do anything to hurt her and so she asked me why my phone was upside down and why I hadn't checked the text - I genuinely didn't realise I'd got a message so I flipped the phone over and it was my friend Becca - the message said "just finished work - you ok? xx" my girlfriend was not happy at all - she started saying I don't understand why this bitch is texting my boyfriend etc etc. She asked me how I knew her and I told her she was just a friend I met a while ago - she was annoyed because I'd never mentioned her and because that was the only message I had from her which meant I'd deleted some - granted I had but I delete most of my messages every week. I told her that I have a lot of friends she doesn't know about and the reason I didn't tell her about Becca was because I didn't want her to think I was cheating - she says that she wouldn't have bothered if I'd told her but I can assure you she would. She told me that she doesn't understand why I am texting any girls at all - even though every single time her phone buzzes it's a guy that she's texting and I never utter a word. Eg we were at the pub before we went back to hers and her friend Jo text kept going on about how they should go to spain one weekend - I got sick of it and just snapped and told her that's ****ing weird and she shouldn't be going on about stuff like that when she's sat with me.

Anyway we hadn't seen each other all weekend coz she was away so we were both really looking forward to the night and to having sex again. She started crying downstairs and I did everything I could to reassure her that she's the one I want and the one I love - I literally couldn't think of anything else to do. We ended up going upstairs and she couldn't sit still, we decided to have sex and while we were doing it she said "im sorry it's not that great tonight but I'm just annoyed". Anyway after that we were lying in bed talking about it all and she told me that she has insecurities about being cheated on and that if she suspected a guy was cheating on her then she would cheat on him - this really really annoyed me and made me panic like crazy, she has huge trust issues with this subject ever since her dad cheated on her mum. Im now terrified that she's going to cheat on me. She then apologised for being quiet and just said she needs time to process it all and decide what she wants. What the hell does that mean?!

I'm just so confused - I genuinely haven't done anything wrong, I would never hurt her or cheat on her - I love her to pieces and she knows this but I think she's making a mountain out of a molehill and now I've no idea what to do because no matter how much I like somebody I won't be made to look a fool of.

I text her at 7.30am when I left hers just saying "Im sorry xx" and she didn't reply - I came out of the gym at 9am and sent her one saying "I hate this - please talk to me x" she responded saying she wasn't awake yet but would text me when she woke up - that was two hours ago and I know she'll be up now because she has college. It does annoy me because it's kinda like she always wants to be in control and the dominant one.

If you guys could give me any advice that would be great - Im struggling here.

Thanks in advance.