Did he cheat on me with his ex?

Right recently a guy I met through a friend told me that my boyfriend slept with his ex girlfriend at a house party. I confronted my boyfriend straight away and he explained to me that his ex was there and they spoke together alone about very personal matters which he told me about and I don't want to repeat as its pretty horrible, but denies anything happening. When I told the guy who told me he got angry and saying I was being a mug and stupid for believing him then one of his mates backed him up. I'm quite lost and not sure what to do or how to sort it all out. By the way the ex cheated on him in the past and she hates me alot and has tried kissing him in front of me but he pushed her away and yelled at her for doing something like that. I just don't know what to think or do and the thought of him cheating on me makes me sick