Why does my fistula start acting up a week before remicade infusion?

It's not the first time I've noticed this. Through level test showed my level was 7.2 in week 5. The therapeutic range is from 3-7. When I reach week 7,both fistula and abdominal pain become unbearable. So, my GI decided I have to take it in week 6. The infusion is due on Thursday; however, fistula pain has started three days ago. I can't figure out the reason

True, it's very confusing. My GI was totally against the idea of reducing the interval between doses and giving me the dose with this trough level. He told me literally bear up the pain and try to push it to week 6. He also wanted the trough level to be a 5 or 6 and not 7. The time before last, he mentioned that I'll too will have another trough level test after 2 or 3 infusions to measure the level again and to see whether taking the dose every 6 weeks was effective or not. After 3 infusions, a year will have passed since I've started remicade and knowing my GI, I think he would like then to perform some scans to see whether the fistula has healed or not. I was put on remicade primarily for the healing of the fistula and not Crohn's. He thinks the inflammation of my intestines is not severe to require remicade. Again another confusing question : If inflammation isn't that severe how did the fistula form? I think these are rhetorical questions without answers.
Hope your trough level test answers your questions.