I am being driven to distraction by DS2 doing this as well. Now 5 months. I'm unwilling to wean as he is, like DS1, atopic and therefore prone to allergies. Plus not really sure it will make a difference. He has just got two teeth, which might have explained it, plus he's had his first cold... ditto. But these issues are now resolved and he should be back to 'normal'. It's very upsetting.
What really gets my goat is that NOWHERE IN ANY PARENTING BOOKS IS THIS MENTIONED. Even in nice, nurturing ones, it's all "3-6 months, your baby may start to be more settled at night now". BULHT! Both of mine slept through occasionally before 3 months, and then were dreadful from 4-6 months. DS1 started going to 5am from about 8/9 months and finally slept through at 1. He's a v.g. sleeper now, though. Just bung him into bed and off he goes.
What is this conspiracy theory though, about the second quarter of baby's life? It really really hacks me off! By now, I'm much more tired, plus a lot of the "falling in love" hormones are wearing off. The last thing I need is unreasonable expectations about sleep. Do they do it to make DH and I fight? Because we really don't need our love tested in this way, thank you very much. And why will the little bugger insist on munching all night, then spend "feed time" in the day chewing on my nipple, pinging it with his teeth and 'forgetting' to let go of it when he turns to look at DS1/radio/car going past/passing cat etc?
[I was whizzing up the motorway and thought "slow down, you don't want to kill yourself and the kids" then I thought "what if only one of the kids died?" Then I thought "Oh please don't leave me the one that doesn't sleep!" He'd better sleep better soon...)
Still, one gurgly smile and big sloppy kiss in the morning and he's forgiven. Luckily.