I know you probably don't want to hear this... but it could be just about anything at that young age! :) Here's a list of possibilities that my husband and I go through every time our little guy (now 9 months old) switches things up on us:
* teething?
* developmental leaps? (rolling over, babbling, crawling...)
* getting sick?
* reaction to something in mom's breastmilk? (like one of the big 8 allergens - dairy, wheat, soy, eggs, peanuts, etc. - or if you formula feed a possible dairy or soy sensitivity)
* hungry? (growth spurt)
* if feeding solids, tummy ache/gas/constipation? and beware of suggestions to start solids this early even though some say they make them sleep longer - they're really not recommended until your child can sit up well unassisted, has a good finger-thumb pincer grasp, and perhaps some teeth - the old, outdated suggestion to start solids before the signs of readiness is still circulating
http://kellymom.com/nutrition/solids/solids-sleep.html
http://kellymom.com/nutrition/solids/solids-when.html
* other physical discomfort, like reflux?
Hang in there! As soon as we think we have something figured out with our kids, and there's a nice schedule that we've grown to depend on, the kids get other ideas and help keep us parents on our toes, flexible, in "detective" mode. ;) Do what works - if this means playing around with sleeping arrangements until you find something where all of you are able to sleep better for a few weeks, go for it. Here's hoping you ALL get some more rest soon!