Constipation in ebf 11 week old

[Char11]

Hi,
Have also posted this in the feeding forum but thought someone here might be able to help too. My 11 week old DS is ebf. He has recently been having Gaviscon (for about 3-4 weeks) for reflux. He normally poos once or twice a day, but had not gone since thurs am (but nappies regularly wet).

This morning, he was quite cheerful then suddenly started crying in pain. I had a look in his nappy and there was a small firm yellow poo in the nappy and some more on the way out. It took him about 5 minutes to finish passing it all, screaming on and off as each new bit passed through. Afterwards I fed him and he was much happier - spent the rest of the morning feeding, sleeping or gurgling at us.

Then about half an hour ago he became grumpy again. Eventually I felt rumblings in his nappy and had a look. He passed a runny poo with harder lumps within it (about an inch big). It was a more greenish yellow with lots of white flecks.

Does this sound like something to worry about. I know gaviscon can cause constipation, and we've stopped giving it. In some ways I'm more concerrned about the second poo, cos of its colour. Could it be a sign of food allergy? He has been tested for dairy but stool sample came back negative. Or is it just his body getting over the constipation? Should we see a dr?

Sorry this is so long...!
Thanks