DS is 6 months old and has been ebf with the introduction of some purees recently.
He has started crying at the start of a feed - by which I mean I offer him a boob, he cries (was not crying beforehand), then realises there is a boob there and latches on and feeds normally.
It sometimes takes a minute or so of upset from him but then he'll have a long feed, so he is hungry.
What am I doing wrong? Could it be teething related? Or has he gone off breastfeeding for some other reason? 
Any thoughts welcome...