Why is my kitten suddenly withdrawn/moody?

I may be answering my own question but I just wanted to get others advice who have more experience with kittens then myself. I have had my new kitten for almost 2 weeks now and so far he has been very sociable, playing, lying by me for cuddles and had seemed very trusting of me. However over the last 2 day he has seemed to swipe me with his claws a bit more aggressively as if not playing and has today been withdrawn and has been hiding under the sofa and not coming to me as he usually would. I took him for his first jabs yesterday and was told that he had fleas so also had a flea treatment today. As far as I am aware I have not done anything to scare him suddenly. Could his behaviour be due to discomfort from the jab still and/or from the fleas? Thanks Claire