Have I made a huge mistake?

So I've spent the last five years of my life in a committed relationship with a man who I adored and we have a little four year old boy together. we had been planning to get married and have been trying for the last year to get pregnant. Everything was going great or so I thought. We recently split up after I found out my partner had been cheating on me and I knew I could never forgive him or trust him again. He has now set up home with this woman and her three children and we haven't seen him since. I feel so terrible that I ever trusted him and it makes me sick to think what he was doing behind my back...well anyway I know I'm better off without him and my future is bright and me and my son will be just fine but... I've recently become very friendly with a man I used to work with. He is a great friend and I think the world of him. He came round one night to watch a dvd and have a few drinks and we ended up getting drunk and having . I don't think either of us expected it to happen. It was just a natural thing and the alcohol didn't help. I woke up in the morning completely mortified and thinking that at 28 years old I should have known better. The thing is because my relationship with my ex had so recently ended and we were trying for a baby I wasn't on the pill and we never used a condom. It is so irresponsible of both of us and I know that but now I'm so worried that I could be pregnant because first of all its far too soon! We only just started seeing each other and I just can't beleive I might be pregnant. I feel like I've made a terrible, stupid mistake when I should have known better!