Hello sir...I wanted to know..I saw my period on...

What about your condition ?
Well, since your last period was on the 12th day, and last intercourse was on the 21st day, there is about 10 days. Normal periods usually occurs every 28 days, so the fertility period is between the 12th-16th day of the period. In your condition, you are 2 days earlier to your fertility period considering that your period happens every 28 days. However, the sperms can live for about 72 days inside the womb and can lead to pregnancy even if the chance is low. 

Recommendation:
There is some meds for such a condition such emergency pills or Morning-After pill which should be taken within 72 hours of having sexual intercourse. Moreover, there is a small device that can be inserted to the womb within 5 days of the intercourse.

Should you take pills ?
You should take it if the intercourse was in the last 72 hours not more than that.
What are the safe days for having sex ?
Well, you should avoid having sex during your fertile period (ovulatory period). 
Ovulatory period:
It's the period in which the ovum leaves the ovary to the fallopian tube. If there is an intercourse during that period, then the possibilty of pregnancy will be high.

When does the ovulation occur ?
In the normal mensural period which occurs every 28 days, the ovulatory period will be between the 12-16 days of the cycle.

Symptoms of ovulation:

• Mild lower abdominal 
• Back pain
• Slight elevation of temperature
• Mood disturbances
• Increase sexual desire
• Increase vaginal secretions