DD is 5mo. She has, typically, woke at 6.30am for a feed, and a bum change then gone back to sleep til 8am. She then manages 2 hours of awake time in between each 45min / 1 hour nap.
For the last few days she has not gone back down after her 6.30 bottle, which is fine, but then even after two hours, she gets grumpy and whiney but still wont go to sleep. The last couple of days, we have put her in the buggy and took her for a loooooonnnnnng walk, which she stays awake for 
She has then had a bottle at 10 and gone down for a sleep around 10.30. Going from 2 hours awake time to 4 hours just seems like a massive jump.
I don't mind, per say, I just find that she is grumpy and hard work for a good chunk of that time and just will not sleep, even though I can see she is tired.