Have been experiencing dark watery bleeding with no pain occasionally in between periods. Started after second period since I had sex, no sex since 5m?

See doctor. Any unexplained vaginal discharge or bleeding needs professional evaluation. There are several potential causes, including STDs, hormonal problems, and uterine or cervical diseases like cancer or pre-cancer. All these are more or less likely depending on your age, nature of your sexual partnerships in the past, etc. Make an appointment today to see your doctor soon. Good luck.