gem, thats absolutely normal

i dont know the english word for it, in german you would say menthal jumps
he does it every few weeks, you'll notice that he is sleeping not very much und he is grumpy the whole day. maybe he drinks not as much as before
it wont be long at all.
after that he can do a lot of new things, grasping for example

if you like i can have look for the weeks, when it happens
one jump is around the ninth week... i think it could be.. he is in the eighth week...

and a few times in babys life they change their sleep pattern without any reason