Could you get preg by having sex after having your period?

I think that the person who told you about the certain time of month was confused. Basically, there is a time of the month (called ovulation) where your chances of becoming pregnant are very high due to the fact you have just released an egg. This takes place (in an average 28 day cycle) around 14 days after the day of your period starting. However, although this is the time you are most at risk, you are also able to get pregnant any other time of the month, even on your period. Because of this, if you do not wish to use a condom you will need to find some other form of birth control for you and your partner, such as the contraceptive pill or implant. Make sure that you are both fully checked for STIs though if you are not using condoms, as this is the only type of birth control that can stop you getting them from each other.

As for your question, yes you could be pregnant. If this happened less than 72 hours ago then go to a pharmacy or your GP as soon as possible and ask for the morning after pill (or Plan B in America). This will bring on another period I believe, but will basically stop the implantation of anything in your womb, so you can’t get pregnant. You can also get a coil fitted within 5 days of unprotected sex, for the same effect. If after this time then as hysterical said, you will just need to wait and then take a test if your period does not arrive.