2 week missed periods, and white discharge from the vagina, leg cramps, and fast heart beat. what could be the reason? pregnancy?

Don’t guess.: Take a test.  they are the same tests used in a doctors office. They can detect a pregnancy 7-10 days after conception. The best time to take one is when you miss your period.  if you doubt the result you can repeat it in 1-2 days. If you were pregnant the hormone level would rise enough for detection.

Pregnancy?: Yes you could be pregnant. Advise you obtain a home pregnancy test kit that tests your urine for pregnancy. If positive then contact your doctor for care.