Every time I masturbate I get pain in the pit of my stomach. Does anybody know why?

Dysorgasmia. Its possible you have dysorgasmia especially if you have this discomfort associated with clitoral stimulation. When a woman orgasms her uterus will usually contract several times during or right after the orgasm. Women with adenomyosis may have dysorgasmia (painful orgasm). Other disorders are possible. I would make sure you don't have a UTI or urethritis as well. See your doctor soon.
Uncertain. This may be related to contraction of the abdominal muscles during orgasm but to be certain, an imaging study such as an abdominal ultrasound could be performed to be sure.