Pregnant??

Me and my boyfriend were having sex and after he came he pulled out and the condom slipped off. The condom was still partially inside of me when it slid off and semen spilled out onto my groin area. I am on birth control but I forgot it when I went out of town for the weekend so I hadn’t taken it for about 4 days when this occurred. I also took a plan B afterwards just to be safe. How concerned should I be that I could be pregnant?