HOW DO I KNOW I AM PREGNANT ? , WHAT ARE THE POSSIBILITIES ? ...

Well, the first thing is - did you have unprotected sex during your ovulation? Ovulation happens for some two days just in the middle between your two periods and that is the only time in the month you can get pregnant because that is when fertile egg is released from the ovary.
Maybe you will get a clearer picture after reading some of these posts:

https://www.steadyhealth.com/Worried_about_pregnancy___Please_read_t72559.html

https://www.steadyhealth.com/__FAQs_on_PRECUM______Please_Read_Before_Posting___t167798.html