Look at this blog it will help you to learn more about your question.
I've got most of my answers there as well, maybe you will too.
P&I generally refers to Principle and Interest, it differs from PITI which refers to Principle, Interest, Taxes and Insurance. I would need more context to know for sure how P&I is increasing your mortgage payment. Your principle payment should remain static unless you have a subprime loan, the interest payment could be changing because you have an ARM, all of this is just a stab in the dark. Post back with more info and we can all try to help from there.
Where did you see "P&I"? Try to give us some context as well.