My period finished yesterday & I had unprotected sex today then I found bright red blood and small clots with lower backpain. Do I need to take Plan B?

Yes. If you have unprotected sex, you must take Plan B although the likelihood of pregnancy is low at this time. When you have sex right after your period ends, it is common for sex to precipitate some bleeding and cramping.

Related Questions

I thought my period finished yesterday and I had unprotected sex today after that I have had bright red blood and small clots with minor cramps?

Post coital bleed. It could just be some residual menstrual flow that hadn't passed yet, but more commonly bright red blood right after sex is from a friable cervix. Friable means bleeds easily when touched. Cramps can also just be from sex itself. Semen has a chemical in it called prostaglandins and that causes uterine cramping. Read more...