Call 0344 811 0811 to report that you're having problems and that you suspect that someone might have cloned your card. (Calls are charged at the same rate as 01 and 02 calls are, from both landlines and mobiles. That means that if you've got spare 'included minutes' on your phone package, they'll be free).
Nectar have had loads of security problems over the years, with some checkout staff being accused of being complicit in cloning cards.
https://www.latestdeals.co.uk/chat/warning-nectar-hit-massive-fraud-again
Their IT staff certainly aren't exempt from being tempted into fraudulent activity either
https://www.bbc.co.uk/news/av/uk-england-12193215