you think you're on to a good thing and then BAM!! It all changes!
This is the most frustrating thing I have found, being a new mum. And I'm sorry, I have no advice! I can only commiserate with you. It was the same here.
Once Natty was a little older (perhaps it was around 6 months?) she was a little more predictable in her sleeping patterns, but even now she surprises me by doing something different every now and again, or by changing her pattern completely (not that this happens much these days - mainly with settling).
Sorry, probably not what you want to hear! But just hang in there. I have found that it gets easier as she gets older.