I menstruated last nov. 7 then we had a sex last nov. 20. Then after 3 days, I experienced light bleeding. I thought I was going to menstruate again. But the bleed was just for a very short period. Please. Help me :(

How long is your cycle? From period to period (start date to start date). It does not matter when it ends.

If you have a normal 28 day cycle you would normally ovulate half way through that (day 14) than 14 days later you would get your period or you wouldn't. (total of 28 days)
If this is the case and your last period was Nov 7th (that is day 1 ) you would ovulate on the 20th. Sperm has the ability to stay active inside us for 3-5 days. There is a chance you could be pregnant. Take a test and contact your doctor to provide accurate results. I don't think spotting 3 days later makes a difference here, implantation takes a little bit longer than that.

How long is your cycle? From period to period (start date to start date). It does not matter when it ends.

If you have a normal 28 day cycle you would normally ovulate half way through that (day 14) than 14 days later you would get your period or you wouldn't. (total of 28 days)
If this is the case and your last period was Nov 7th (that is day 1 ) you would ovulate on the 20th. Sperm has the ability to stay active inside us for 3-5 days. There is a chance you could be pregnant. Take a test and contact your doctor to provide accurate results. I don't think spotting 3 days later makes a difference here, implantation takes a little bit longer than that.

Sometimes my period is at the last week of the month, but my last period started 2nd week of the month. Is still there a chance of pregnancy even when my boyfriend widraw it?

If he used the "pull out" method there is always a chance as men pre- ejaculate before he is able to withdraw. Consult your doctor for a blood test to find out for sure. Blood test are accurate show results earlier than a home pregnancy test.

When having unprotected sex, there is always a risk for pregnancy.

Define short period. There is the possibility of odd periods. Sometimes very light or very heavy depending.

Withdrawl isn't a method of birth control. It doesn't control anything. The chances are still high that you will get pregnant.