I 2nd the fact that if they accuse of you of cheating, then they are most likely cheating. My ex accused me several times. I never cheated on her. But she cheated on me twice that I know of and also propositioned our landlord for sex when I was out of town on business for a week.
I actually had my ex throw that same way of thinking at me. The last I saw her was early January 2013. There was always an excuse: she didn't feel good (for one reason or another), she was at her mothers, she didn't feel like going out, we weren't getting along (she actually told me if we began to get along through texting we could move up to phone calls... .what the hell was I doing believing this nonsense?), etc. and she decided she didn't want to be involved with me anymore yet still found it enjoyable to control certain parts of my life. I begin to get a little suspicious given that she was always able to tell me she loved me but always seemed to have something other than time for me lined up on her weekends. I asked her, in a polite and non-confrontational way, if there was another man in the picture. I gave her my reasons for thinking so and she gave me the old deep sigh and 'There you go presuming again'! It was this that gave her the basis to tell her friends I was insecure. The 'friend' of ours I speak about? I told her that yes, I was insecure, but that stemmed from her continuously telling me she loved me, controlling the tempo of our conversation schedule, controlling what women I could have on my Facebook friends list yet when it came time for her to prove her feelings, she always had an excuse. So I began to wonder if there was another man. My ex's reasons for her insecurity are already hot wired into her brain. That's how she is. I had that confirmed back in May by way of this 'friend' and her dishing all kinds of dirt on my ex. I wasn't insecure about anything until I begin suspecting something. I had no reason to be. I'm sure anyone else would've felt a bit insecure, too.
In late March, I finally had enough and changed my #. She went berserk and went as far as asking a close lady friend of mine for it, and she declined to give it to my ex. So she deleted her from Facebook and essentially told her 'You have your BFF back now. I'm done caring!' Then the emails started pouring into my inbox practically DEMANDING I give her my new #. I asked why she couldn't even give me a few hours on any Saturday night for the past 3 months and the excuses came pouring out. Then she said 'Well how can I go out with you if I don't even have your new #? Oh sure, I'll be glad to go out with you even though I don't have a way to get a hold of you!' I told her we'd been getting by just fine by typing messages back and forth like we've been doing since the beginning and she could leave her email open on her phone, as would I, and we could communicate that way. I eventually gave in, as usual, and gave her my new #. She called me and I asked her again if someone else was in the picture. Again, I stated why I felt the way I did and she came back with 'You know, you've asked me this before. Maybe you're the one that's seeing someone behind my back and your guilt is causing you to ask me if I'm doing anything behind your back.' I laughed and told her she had more nerve than the law allows to use that excuse against me when I was the one making effort after effort every weekend to see her and she could never give me a few hours of her time on any Saturday night. I was the one that made time, every weekend, only to hear one excuse after another. How I let this go on and did nothing for so long boggles my mind now.
Typically, when someone just starts asking you things about their faithfulness to you and starts acting weird all of sudden about where you're going and who you're with (even though you've been going to the same places with the same people) then it's a certainty they feel guilt over cheating and figure by accusing you, it somehow validates them doing it. A very, very demented way of thinking and allowing a made up thought give them the green light to cheat on you.