What is the principal amount you borrowed from lender A? What is the interest rate on your offical loan document? You can figure out yourself which party is wrong.
For example, if your principal is $200,000, and interest rate 6%, then your interest in the 1st month is $1000. If you pay only $500 per month, your principal will grow. This is called negative mortgaging.
Before you decide to sue anyone, you should ask a loan agent to check your loan document to make sure your understanding is right. He or she will charge much less than a lawyer.
Good luck!