Boyfriend Came Inside Me. Can I just start Birth Control?!?

My last period started on 10/13/17. He came inside me on 10/21 and 10/22. I have incredibly regular cycles (down to the day), so I shouldn't be ovulating until 10/27. I know that sperm can live 3-5 days, but if I start my bc today, I won't ovulate on the 27, right? I really don't want to buy Plan B bc... show more My last period started on 10/13/17. He came inside me on 10/21 and 10/22. I have incredibly regular cycles (down to the day), so I shouldn't be ovulating until 10/27. I know that sperm can live 3-5 days, but if I start my bc today, I won't ovulate on the 27, right?

I really don't want to buy Plan B bc it's sooo expensive.

Please don't say "you're obviously too stupid to be having unprotected sex" because I already know unprotected sex is stupid. If you could save your judgment and just answer the question, I'd really appreciate it.
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