I've read this thread before and felt so sympathetic towards everyone experiencing this. Now I am going through it myself and I want to cry with tiredness.
DS is 14 weeks and has always been a very straightforward baby. He would go down awake, self settle, wake for a feed, go straight back to sleep. At 10 weeks he started sleeping through 7-7 with a 10pm dream feed. Then at 12.5 weeks it all went wrong. To start with I though it was teething as all the symptoms were there, but then the teething symptoms stopped and the sleeping hasn't started again. I have no idea whether this a growth spurt, early four month sleep regression or something else entirely. During the day he is a delight but at night he is like a different baby.
He is definitely hungry at night, which I can cope with, but he is now very unsettled. Saturday night was a new low, he woke every half an hour throughout the night. He isn't crying, but grizzling and fussing and if I don't offer him a dummy, put the dummy back in again endlessly, stroke and shush he will start crying.
Last night was slightly better, he woke at 11pm-midnight, 1:30am-3am and 4:30am-5am but lots of fussing in between times.
I am back at work two days a week and the thought of having to cope tomorrow on so little sleep makes me want to cry.