Dwp want me to repay uc.

Basically I was between jobs last October to December, panicked as was worried I would be unable to pay my insurance etc. So signed on, whilst I waited to restart my old job in December. Which was a given I'm back there working.

However this week got a letter saying I owe 200 to dwp, that was from my uc, that's the only payment I had, why am I being told to repay this,, seems odd ?