After being a fairly good sleeper, my 6 month old ds has gone to waking up twice in the night for milk. He has a about 20oz during the day split into 3 feeds and he very quickly established himself on 3 meals a day. He attacked weaning with gusto and relishes a variety of foods. I don't understand why this isn't seeing him through the night - he seems hungrier than ever at 11pm!
My DS has just started doing this too, he's teething quite badly but I"m sure he's having more milk now he's weaning. Babies are funny, I wouldn't worry, it might be just a growth spurt? 6 months is meant to be the time for it. DS has just come out of a phase of waking up 3 times a night back to his normal 5am feed.