My DS is 9 wks and did kicking, straining and the red face thing from about 2 weeks onwards. He did it even although he's always been regular. It was particularly bad at night and we couldn't sleep with all the grunting and straining. He wasn't actually crying though, just noisy. He is also ebf, tracking 50th percentile.
My HV said he had just discovered he could grunt and was doing it to entertain himself. Which sounds a bit far fetched. She also said, not in so many words, that we were paranoid new parents and we'd relax about it over time.
I also read online that babies can't co-ordinate their poos properly and were going through the motions, so to speak, while they worked out how to go. Which sounds a bit more plausible.
Infacol didn't make that much difference with us.
We never figured out how to stop it but his straining and grunting is a bit less frequent, although he still keeps us awake sometimes - although my HV is right in that we did get more used to it. Otherwise he's doing fine.
I mentioned it to the dr as well and she didn't seem concerned at all. I think some babies just grunt and strain, although if he still doesn't poo you could try asking your HV?