Sleeping through but refusing to nap in the day?

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DD (8 months) has slept through about 3 times. She is breastfed and when she doesn't sleep through she generally wakes twice for a feed. She has always been a bit crap at napping and has never, despite my best efforts, had much of a routine during the day - we aim for rough times but don't always get there! Her naps can range from 30 mins to 2 hours (rarely!) and she has at least two naps a day, probably 50% of the time she has a 3rd. She has a set bedtime routine and generally self settles to bed with minimal crying. There has always been a bit more crying for naps, but never real wailing, and usually less than ten minutes before she falls asleep.
She slept through last night, 7pm until 6am, then fell asleep again feeding so didn't wake for the day until 7.30 (hurrah!). However, she has wailed like a BANSHEE both times I have tried to get her to nap today. Like never before!! I seem to remember that naps were also tricky the day after the last time she slept through. She ended up sleeping for around 30mins at 10.30 this morning then I tried to put her down again around 13.30 (when she rubbed her eyes) but it's taken me until now (45 mins) to get her to nap. The only thing that's worked is feeding her to sleep (on my second attempt!).
Anybody have any idea why this is - could it be related to sleeping through, does she just feel more rested today? Or just one of those days and a complete coincidence?!