I work for a financial planner, so I asked him what reasons a mortgage payment would increase. Most common reasons are
1. Its an adjustable rate mortgage and rates have gone up. Rate increases are documented in mortgage paperwork.
2. Its a structured payment for a jumbo mortgage. Very large mortgages have structured payment increases to pay down the loan. The increase amounts and dates are documented in the paperwork.
3. Change in costs paid by escrow through your mortgage lender (usually property taxes and homeowners insurance)
4. Repayment of late fees/missed payments. Payments that were missed can be repaid over time by adding payments to current monthly payment. This increases the payment until the past due amount is paid off.

A mortgage statement will tell you quite a bit. Loan type. payment breakdown, past due amount, etc). It may be helpful to find an older statement to compare so you can see where the differences are in the payment breakdown. If your escrow payments have dramatically increased, its possible that property taxes have increased. If the interest payments have jumped, you likely have an adjustable rate mortgage or ARM. If the principal amount has increased, its likely a structured change in a jumbo loan. Repayment of missed payments and fees will be a separate line item in the payment breakdown.

I can't take away the stress this situation causes. I hope that the information helps you.