The Shulhan Aruch (Orah Haim 328:3) rules that if a person experiences severe internal pain on Shabbat, this is considered a potentially life-threatening situation which must be immediately treated even if this entails Shabbat desecration. Regardless of whether the source of the pain can be identified, the individual is considered to be in a potentially life-threatening situation, and therefore, everything necessary must be done to treat the patient even at the expense of the Shabbat prohibitions. Even if the pain is not severe enough that the patient needs to lie down, as long as he experiences strong pain from inside the body, treatment must be sought even if this entails violating Shabbat.
This is the ruling of Rav Shlomo Miller (contemporary), in his work Shabbat Shlomo (p. 232; listen to audio recording for precise citation).
