formula is not all it's cracked up to be...
my previously wonderful sleeper(now 7.5 months) has been waking around 3 times per night for the past 3 weeks..incidently, she was exclusively breastfed for hte first 6 months. we introduced formula at that point.
keep in mind they say.... growth spurts are; 3 weeks, 6 weeks, 3 months, 6 months...
maybe yours is just experiencing a growth spurt. don't worry, this too shall pass!
good luck!