DD is 7 1/2 months, is established on 3 meals and 3 bottles (formula) a day. She was fine until Tues when she tried to poo, started crying and then passed a really big hard poop - I had to hold her legs up etc. to help her.
Nothing yesterday, then a bit of leakage this morning so I thought she might be constipated - I gave her a little OJ and did some tummy massage, she then started to strain so I took nappy off, held her legs up and she did another really hard poo and screamed the whole time 
She currently eats a mixture of pretty much everything (protein, fruit, veg, root veg, bready things etc.)and has done since we started to wean ( she was weaned at 6 months so went straight onto finger foods, with the occasional puree).
The only thing I can think of that's changed is that I've been using 'hungry baby' milk this week as the shop had run out of the usual stuff - could that be it? Or any other ideas?
TIA