Welcome to HCM
We understand your concerns
I went through your problem description. I suggest you not to worry too much. Masturbation
is not a sin. Masturbation is normal practice and does not cause any harm to body if done moderately (maximum 5 times a week after the age of 20).
You are anxious or apprehensive about something which causes your stomach tightness while thinking about sex. I presume that it is the fear about masturbation. Forget it.
Masturbation cannot cause these symptoms physically, but mentally it can. Consult a psychologist
Hope this clears your doubts. Available for further clarifications.