Why has this happened after 8 months of successful breastfeeding?

[Pointlessfan]

Been breastfeeding DD with no problems for 8 months. Starting to wind down ready to go back to work so DH has been giving her formula at bedtime (6.30 pm ish) instead of expressed milk. After a couple of uncomfortable nights, supply has settled down, been fine for last 2 weeks.
So I've been feeding DD at about 4pm then she has bottle and usually sleeps until about 4 before waking for a feed.
Today I fed het around 5pm. It is now 10.20 pm and one boob is all engorged. Hard, uncomfortable and a funny shape. Just tried to express but didn't get much so it hasn't really helped. I'm worried about getting blocked ducts if DD sleeps until 4, she sometimes sleeps even longer.
Any ideas why this had happened or what I can do about it? I've already had a beer as I heard alcohol can reduce milk supply!