You can also say ‘I have sharp pain in my leg.’ They are all fine with similar meanings. The article ‘à’ makes it sound like there is just one noticeable pain that I feel. The others suggest that it isn’t constant or that it isn’t perceived as being the same all the time. I agree with the other respondent who said that ‘a sharp pain’ would be the most common to hear.

"Pain" is used more frequently than "pains" in most English text. While both your sentences are correct and valid, the second form is not as common. The most frequent use of "pains" as a noun is with the phrase "growing pains" which describes someone who's going through growth, but also has pains while they are growing.