nationstar mortgage bought my mortgage from BOA at end of 2013; my fixed rate mortgage has gone up $100 (>10%) since they took it over. dallas texas

I just saw a report by someone else who had the same experience, and I feel certain there are more of us out there.

Bank of America sold my home loan to Nationstar at the end of 2013. Since they bought it, my monthly payment has gone up considerably. They claim 1. BOA never put enough in escrow so I was short when they bought it, 2. Taxes increased and 3. (And I would sure like to know if this is legal), Nationstar requires escrow accounts to always have a certain amount of $ over and above the actual amount; supposedly my account was short @$120 when they took bought it,  PLUS, they told me, they require a minimum of $200 or $250 (an extra month or some percentage--don't remember what she said) in your account at all times, which BOA does not do. That added about $300/year. 

First, is this legal--to have the company that buys your loan add new rules about escrow? Second, I still can't get them to explain how that additional amount/year translates to an increase PER MONTH of @$100 since they bought the loan.

I have owned my home since 2003 and refinanced twice--once, before it expired,  to get rid of ARM (2006) & a second time (mid 2013)  to obtain a lower rate and have payments below $1k. I do not believe my payment increased in 10 years--at least not until Nationstar took over.