Are they, like, Halloween coloured ones? If so, they taste different because the are a new batch. M&M's have a shelf life of one year before they are bought by the consumer on average.
My mum and I one year called the company about our batch of M&M's that tasted different and they said the chocolate ferments, so that is the flavour of M&M's as we know it. The newer batches that they make specificaly for holidays are not fermented, thus taste different.
I don't really like the peanut butter ones though peanut butter is one of my favourite foodstuffs.