J.F.
Looks like other moms have already suggested growth spurts. I would agree with that.
Many first-time parents (including me with my first baby) think that once your baby starts sleeping through the night that they should then continue to do this forevermore and that there is something wrong if they don't. The truth is that babies go through lots of changes the first year (and beyond) and their sleep cycles will change along with growth and development and emotional need. The best thing you can do is be flexible and let her tell you what she needs. I have discovered that since babies sleep patterns change so much the first year, it's just easier for me to have my babies sleep with me. I sleep better and so do they. When they wake up, I just roll over and nurse them and we both just drift back to sleep. Wish I would have done this more with my first baby.
Love that little one and enjoy these precious fleeting days!
J. (mom of 5)