Hi,
It can be due to
1. 
Urinary tract infections with increased frequency of urination, irritation during urination, lower 
abdominal pain - urine culture will help to diagnose. 
2. 
Urethritis - damage to the 
urethra from 
masturbation, irritation from a product, such as soap, deodorant, or detergent, adenovirus, genital injuries.
3. Candida infection can also cause watery discharge with itching. smear from the discharge will help you diagnose and start early antifungal medication.
4. Hygiene issue - make sure your hands are clean before masturbation. If you are using vibrators or dildo, make sure they are sterilized before every use. 
It is better you get It checked with your gynecologist in person.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Dr. S.R.Raveendran, Sexologist