My DD changes all the time. At 7 weeks she was sleeping from 8- 4:00 then the time kept getting shorter and now at nearly 4 months she won't go longer than 4 hours! I found we never were consistant can change from one day to the next!
It is easy to feel dissapointed though, I was getting used to that big sleep she used to have and would feel pretty goo the next day and now she's waking more often a night it hit me hard, I feel exhausted all day!