Quick Question

Hey all! I'm just curious if anyone can maybe answer my question! See, I had sex Sunday, 09/13/15 with my bf and today I have lower back pain. I also had egg-white cm yesterday, Monday, cuz it's Tuesday now. Any ideas as to why in having lower back pain? It's not severe it's just there. I also will add I finished my period on the 9th. Thanks, ladies!