My son is going through the exact same thing! He is 9.5 months now, it started a few weeks ago. He would have his last breastfeed at around 7:45 and sleep from around 8-8:30pm and wake around 5-6 am, has milk then sleep till 9-9:30am. Now he goes to sleep at the same time but wakes at at any hour during the night...sometimes every 3 to 4 hours. Then if he is awake at 7 he doesn't want to go back to sleep! And then some nights he will sleep through again.
Apparently their sleep patterns change as they grow. This article may help:
Why is my baby waking in the night?
I breastfeed him whenever he wakes up as it's the best way for us to settle him, then put him down and he goes back to sleep. I can't wait till this is over then I can get a good night's sleep again!
Hopefully your baby will get back his normal sleep routine or you'll be able to find something that works for you and baby.