Pattern to the pain/effects of chemo?

My husband had his second dose of FOLFIRINOX last week, so this is Day 7. About Day 6-7 last time, he also had this "awful" sensation. Not pain exactly, but very low in his abdomen he says it feels awful, uncomfortable, like it's disintegrating. This sensation faded last time, so we're hoping it fades this time. He has had the CPneurolysis, so I'm wondering if he would be having other pains, but the CPN is blocking them. His CA19-9 went from 3700 to 2900, taken on Day -1 of each chemo cycle. So we're hoping he's responding to the chemo treatments.