My dd1 was 19 months old when dd2 was born and right around the time of the baby's arrival, her sleep was HORRID. She was up every single night for 2+ hours and would not go back to sleep. After dd2 arrived, it continued for about another 2 weeks, and then she was fine. The only thing I could ever connect it with was the baby -- and everything was very positive for her, so I wouldn't blame yourself at all, it's just that I think they can sense change when they're so little and don't have the words to deal with it all during the day. We just basically slept on her floor until it passed -- well, mainly dh slept on her floor as I was either very pregnant or nursing dd2.