My 11 month old daughter went through this, and it coincided with her going in to her own room. It was also around the time she started nursery, so she was constantly getting coughs and colds and viruses. She was also cutting teeth and had horrible separation anxiety.
She went from being a 9pm to 6am sleeper (straight through) to getting up anything between 2 to 6 times per night. I was broken and on a fast track to antidepressants.
Then, just as suddenly as it all started, it stopped. She now sleeps soundly from 7pm to 6am with only very occasional wakenings and if she does wake, it's only once. We didn't do any sleep training or any CIO - I always got up to get her when she was upset. Not that I'm judging because now, after being so sleep deprived, you do what you have to do to get some rest.
In her case, it went on for 12 weeks a while, but it did get better.
However - I'm still getting over the trauma of it, and my body still wakes up half way through the night purely because I got used to functioning on no sleep, so now if I've slept more than four consecutive hours, I am raring to go. Which is no good at 4am 