OGS uses the Glicko-2 rating system, which is a little bit more volatile than the ranking systems that most other servers use. This is on purpose (as far as I know) and has been discussed a lot already in these two forum posts:
Apart from that, your rank on KGS is not directly translatable to your rank here. As you can see in these results, KGS 13k corresponds with OGS 15k. Unfortunately there was no data about lower rated people, but there has been another comparison in 2015, which can be found on sensei’s library, which would correlate KGS 15k with an OGS 18k.
I see you also have a bit of a losing streak going on, which of course has a negative impact on your rank, which could explain why you’re a couple of ranks lower than you would expect. Don’t worry, because Glicko has you covered here too: if you win a few games, you will also gain rank rather quickly.