Stomach acidity? If a person feels either acidity in the stomach or a sour taste in the throat after an activity, he may be having some stomach reflux (may lead to heartburn). Activities that worsen reflux include lying down, doing sit-ups, strapping on tight clothes or tight seatbelts, etc... A primary care doctor should evaluate the symptoms. Maybe the bodily contractions in orgasm are causing some reflux?
Re-ask question. I am not sure anyone can answer your question based on how you asked it. Don't understand "feels like acidity". Be more specific. If you mean that your penis feels like it is burning after masturbating, then this is probably the effect of the mechanical rubbing effect on the urethra (urine tube) and it may be irritated and burn.