IBS worse because of new stomach medicine?

My gastroenterologist has just recently changed my Nexium to Pantoprazole for acid reflux. I did notice my stomach symptoms improved, but I started having constipation which is not listed as a side-effect. I asked my doctor and she said to take the medication at night, but didn't mention any side effects. Since then, despite the constipation, my IBS has gotten worse with going to the bathroom 4-5 times a day, abdominal pain, and nausea. Anyone else experience this? Thank you.