I am in my period. My boyfriend let his sperm inside of me. He said if I'm in my period, I won't get pregnant. Can I get pregnant or not?
Anytime a couple engages in sex, there is a possibility of pregnancy. After all, one of the purposes of sex is to produce children.
Engaging in sex when you are not married is loaded with problems. You run the risk of sexually transmitted diseases because it is highly likely that neither of you are monogamous. Solomon speaks of the regret that comes from promiscuous sex by saying, "And you mourn at last, when your flesh and your body are consumed, and say: "How I have hated instruction, and my heart despised correction! I have not obeyed the voice of my teachers, nor inclined my ear to those who instructed me! I was on the verge of total ruin, in the midst of the assembly and congregation"" (Proverbs 5:11-14). If a person is willing to have sex with someone they are not committed to in marriage, they are almost always willing to have multiple sexual partners. Such a situation increases the odds of gaining a sexually transmitted disease dramatically.
Sex can lead to the conception of children. Unfortunately, in this day women are far too casual about pregnancy. Many will have an abortion, killing the innocent child, solely for the convenience of the mother. Hence, not only is a person committing fornication (the sin of having sex outside of marriage), it contains that possibility of leading to the sin of killing the innocent. God warned Israel, "do not kill the innocent or the righteous, for I will not acquit the guilty" (Exodus 23:7).
Even when children are permitted to be born, they face a hard life growing up in an unstable environment where mom moves from boyfriend to boyfriend -- seeking "love" in the wrong fashion and never finding it. It has been document that children are at higher risk for physical harm when the people they are living with are not their parents. The same disregard for societal norms and religious demands for marriage also lead a person to have a casual disregard for the welfare of children.
In regards to your question: yes, it is possible for a woman to become pregnant even when sex takes place during her menstrual period. In order to understand this, you need to know that sperm can survive up to six days in a woman's body. A woman can only become pregnant on the day she ovulates, but she can receive the sperm prior to her ovulation and still become pregnant. Ovulation (the releasing of an egg) occurs two weeks before a woman's next period. The problem is that many women are not absolutely regular in their monthly periods. You cannot say with absolute certainty when your next period will begin. It might be as long as 32 days or as short as 21 days. The average is 28 days, but many things can cause a woman's cycle to vary.
Now, let us suppose that this month you just happen to have a short cycle of only 21 days. That means your egg is released just after your period ends. If you had sex within six days prior, there is a possibility that sperm will be present in the fallopian tubes to met the egg and cause pregnancy. The chart below illustrates this:
By the way, trying to time sex to the period after ovulation is still iffy. There is always the possibility that you will have a longer than normal cycle, putting you in range of getting pregnant.
This is why married couples who try to use the rhythm method (engaging in sex only during the period of the month when pregnancy is least likely to occur) still face a 20% chance of becoming pregnant in one year.
Wouldn't it be better to stop playing games with your life and the potential lives of your children? Do things in the proper order: find someone you want to spend the rest of your life with, fall in love, get married, and then enjoy sex without the constant worries and sinfulness of uncommitted sex.