U.K. vaccinations help!

Hi, my DS is 3 months old and just had his 2nd lot of vacs Thursday. He's been a little grumpy since but his eating is what has been off. He usually sleeps the whole night but has woken up last night and the night before. Last night he woke at 3am and took 6oz (he's a BIG baby) and this morning I've only just managed to get 2oz down him, he's In a really good mood and is alert etc but just showing no interest in milk. Yesterday was similar. It's just usually he wakes up starving so I'm not sure if it's because he ate in the middle of the night (he used to and still eat breakfast) should I be concerned or is this normal? Sorry I'm a FTM, thank you x