Bizarre credit card statement from Chase! Why?

I originally did a balance transfer of some $13,500 in private student loans from Sallie Mae to my Chase credit card. Sallie Mae was charging me 14.9% variable and Chase offered me a weighted average interest rate of 4.22% fixed for life, so long as I make payments on-time. Well, I've been paying steadily and that balance has shrunk to $9,882.14. I remember my first minimum payments being in excess of $250. Now they have come down to less than $200. I still pay them a fixed $250 per month. But, this statement cycle I received a statement which said I had a minimum payment of $0! Yeah, that's right, ZERO. I immediately thought, "Huh? What the heck?". I figured that I had missed a payment or double-paid or something. I checked my account history and everything was okay. So, I called Chase and the lady on the phone told me that since my previous payment exceeded the minimum payment that I wouldn't get one this month. But, I always pay above the minimum payment unless the APR is 0%. Why did they cancel my minimum payment for this month? Needless to say, me being a skeptic, I sent them another $250 anyway.