Confused and highly upset.?

I met this guy at a frat party and we never talked too much. We just went out to eat. 3 months later I saw him at another party, and had a one night stand (please don't judge me). I met another guy in the same fraternity that night but we didn't start talking until 3 days later. While we're not in an official relationship, just dating, the new guy and I have been kickin it for 2 months now. In the beginning he asked had I been involved with any of the fraternity brothers, not necessarily in his chapter but period. I told him about one guy but not the one I had a one night stand with, which was before him. Why on earth would I tell a new guy I'm interested in about a one night stand? That's wack. Considering he slept with someone after we started dating and didn't tell me anything until I asked yesterday, am I wrong for not telling him? Or is he just being a douche? HELLLLPP!!!