Pregnancy??

So yesterday me and my boyfriend had sex (with condom) but after he took it off he ejaculated on my back just at the top of my bum. He washed it off and said it had gone but now I'm paranoid as this hasn't happened before. I'm on the pill and I'm on my second week of my current strip (been on it for about a year - three weeks of pills then a 7 day break). But I noticed a little blood in my underwear after, and a small amount today as well. Paranoid about pregnancy.. Advice??