Periods don't always need to be regular. Irregular periods can occur for various reasons, even after sex and ejaculation. Bleeding three days after sex could be implantation bleeding, stress, hormonal imbalances, or even pregnancy. If you are nervous, you can take a pregnancy test. To help regulate your periods, stay healthy, eat well, and manage stress.

there are SEVERAL reasons for delayed menstruation in teenage girls. Puberty timing varies widely. Hormonal imbalances can delay menstruation. Excessive exercise or underweight can impact menstrual cycles. Stress or medication can also delay periods Consult with a DOCTOR.. 

Yes, you can get pregnant after your last menstrual period on 16 Jan if you had an intercourse on 8 Feb, the chances of which largely depend on the timing of the ovulation and menstrual cycle regularity. If you have any worries about pregnancy or reproductive health, then please see the gynecologist for an examination and tips.
 

In some cases, the concern could lead to delays in periods. Another reason for late periods might be pregnancy or hormonal changes. The symptoms of pregnancy include nausea, tiredness, and breast tenderness. On the one hand, you can do a home pregnancy test to confirm the pregnancy.