Me and my boyfriend have been having unprotected sex but he has never ejaculated inside of me but now I am 3 days late for my period and I'm worried because I have been reading of the possibilities of pregnancy by pre-ejaculation. I have been experiencing breast sensitivity, diarrhea, and stomach pains. Am I pregnant? Should I be worried? Or is my stress about being pregnant delaying my period?

I would be scared to death personally. Before you had sex is when you needed to learn about sex education.
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Yes of course you can become pregnant that way. Next it will happen that he will not always pull out in time also.

Even with condoms there is a 3 to 10 percent failure rate.

So yes, perhaps time to start picking out baby names with boyfriends, finding out if he wants to be with you to tell your parents.

You need to wait about 10 days and if still not period, do a pregnancy test with first urine of the morning