My DS weighed 13.1kgs at his 2 year old appointment the other week but I had weighed him on my scales a couple of weeks earlier and he weighed 13.5kgs. I'd noticed a quite a bit of fluctuation up and down in the 3 or 4 months prior (he likes standing on the scales while he is waiting for his bath to run - I'm not paranoid LOL) so your DS' weight on that day may have been just a bit of an aberration, just like we can get.

However, I think it may be just a bit of a growth spurt, especially if he's grown half a centimetre in the last 3 weeks, that's quite a lot at this age. I personally wouldn't worry too much unless you think he's been eating food that's not good for him lately...my MCHN used to say that as long as a kid was somewhere on the chart within the top and bottom percentiles she didn't worry too much about weight. My DS has ranged back and forth from the 25th to 75th percentiles and everywhere in between since he was born.