It could also be a developmental leap. Google "wonder weeks". There is one around 8 weeks when bubs tend to get fussy and sleep poorly again. There's not much to do apart from riding it out - it will eventually pass. DD started sleeping for 5 hours at a time over night at 6 weeks. Then it all went out the window at 8 weeks and has only just settled in the last month (only because we started co-sleeping).

It may be reflux as well. DD had this so I kept her upright for at least 30 minutes after feeds and propped the head of her bassinet up with a phone book.