INCOME RELATED ESA TO NEW STYLE ESA

I received my transition letter from DWP late last year. I just couldn't cope with the online form & having savings that would last me for a few months I rang them to say I wouldn't be claiming UC as couldn't deal with the stress. I was told my ESA would end in January. Letter and P45 received in January. P60 in April.
Thought it had stopped. Didn't check bank (I'm hopeless at keeping track etc).
Fast forward to this week to my shock they are paying me New Style Esa fortnightly. Panicking I added it all up & reckoned I owed them about £2500. Plucked up the courage to ring esa today to stop payments as UC told me my benefits would stop, money is still in bank as I didn't realise they were still paying me.
Have been told (I asked for 100% clarification 3 times & read letter out from UC) my national insurance contributions have been looked at from 2009 to 2011 and all is correct and I'm being paid correctly nothing to worry about?
Has anyone else had this? Didn't need to claim UC at all as its purely for a top up? Will be paid until retirement unless a change of circumstances. All the stress of UC claims for a mere £16 a week top up payment. I had a witness to this phonecall just in case.

Thankyou David. It was strongly worded in the letter and by phone call to UC that everything would stop hence getting a calculator out and sheer panic that I'd done something wrong. When I rang them to repay I was quite shocked and not entirely convinced, hence my question. My outgoings are so minimum at the moment, I'm lucky enough not to need the £16. Apologies to anyone if it came across the wrong way. I was trying to get everything down.