Could you provide me with verses for nullifying magic, and could the symptoms mentioned (stomach pains, dizziness, and the absence of organic disease after examinations) be an indication of magic?

The entire Holy Quran is a healing and a guidance. A Muslim should dedicate a portion of their time to reading it, for the Quran is a healing for souls and bodies if its commands are obeyed. For healing from magic or the evil eye, one should regularly recite Al-Fatihah, Ayatul-Kursi, the concluding verses of Surah Al-Baqarah, as much as is easy from Al-Baqarah, Al-Ikhlas, and Al-Mu'awwidhatayn. Additionally, one should recite the verses that invalidate magic in Surah Al-Baqarah (102), Al-A'raf (117-120), Yunus (80-82), and Taha (69). These verses are proven, by Allah's permission, to be effective in healing from magic.