I'd say you need to start introducing safe IF NOW! The BTC diet can only be done for 3 days MAX if you are C. So 2 days might be all your body can take before the C starts getting worse from no IF. Introduce it very carefully - think cooked and pureed IF veggies (NB. not peeled as that actually removes the If instead of just making it safer to digest).
Tho please check the fat content on the crisps...I assume you've already checked them for triggers?