Be careful with too much fiber in the beginning. Start slow and add more gradually.
There are two types (of natural) fiber: soluble and insoluble.
Insoluble fiber sources are: dark green leafy vegetables (spinach, chard, etc), nuts, fruit and root veggie skins.
Soluble fiber sources: nuts, oats and oat bran, dried peas and beans (cooked of course), oranges, apples, carrots
Fiber helps move things along in the colon but too much in a weak colon can cause constipation and other annoying symptoms.
And I totally agree with subdued about
keeping a food journal. Everyones gut is different, get to know yours