Should I be worried? DD (15wk) has spent about 35 - 45mins in total (sometimes up to 2 hours in an evening!) feeding from both breasts for a while now but yesterday she seemed reluctant to feed after about 10 - 15 mins and refused second breast, and she is still at it today. She comes off happy and doesn't seem hungry again for about 3 hours, didn't sleep any differently, still filling nappies, so I'm a bit perplexed. I fed DS for 13 months so I'm not a complete beginner at bf but he just slowly dwindled down to fast feeding as he got more efficient - certainly not overnight. I know they're all different, but this has surprised me.
If everything else seems normal for her (and only you know if that's the case) it doesn't seem particularly unusual behaviour to me. Could be just a phase - the 4mths-ish mark is a notoriously fussy time for lots of babies (neo-cortex kicking in can freak them out) and that manifests itself in lots of different ways - do you remember if your DS changed his behaviour (eating, sleeping, general crankiness) around this time?
Not wishing to be a profit of doom, but may be slightly under the weather, or working up to a bit of a cold..?
As long as still having plenty of wet nappies, and no other symptoms (temp, rash, you know the kind of thing) it just sounds like something to keep an eye on, particularly as she's still asking to feed, even if she's not on for long .