Ibupforen alone can cause stomach pain, especially considering that you have an empty stomach on SN protocol. NSAID (ibuprofen, aspirin, diclofenac etc.) type of meds usually irritate the stomach when taken on empty stomach. It obviously depends on your body, if you have pre-existing stomach problems like reflux it is more likely to cause stomach pain. For example I know for certain that my body tolerates paracetamol better, so I would go for that if I were to take SN. Paracetamol is relatively weak as a painkiller, but it works good enough for me for headache.