If someone's stomach hurts on Sheva asar b'tammuz, should she fast? Or do these extenuating circumstances free her from her obligation?
On 17 Tammuz, like on Tzom Gedalyia and 10 Tevet, the law is that
One who is ill, even if his life is not in danger, should not fast (Kitzur Shulchan Aruch 121:9)
adds the Mishna Brura (550:4)
Indeed it is prohibited for one who suffers greatly by the fast to be stringent upon himself and fast.
As to what considers "ill" and "suffers greatly", one should ask his or her Rav, who can ask further questions and personalize an answer for the particular person.
Refua shlema !