FNB Secret charges

FNB is one bank that will have secret charges that will cost more than your monthly fees. 2nd time in a row I have been charged a excess limit fee that I know nothing about. It's a ****!! My account went over not because I went over limit but because there were no funds in my account to deduct the monthly fee and last month was an amount from a toll gate I went over with. I was charged 400 more. I don't agree with this and want it looked into