oily mouthfeel "trappist"

I recently brewed a trappist style beer with pilsner 13% weat malt 32% and pale ale malt about 39 % and some speacial b and 500g of candy sugar.
OG=1.080

fermentation(wyeast 3787) around 19-20 degrees celsius was starting explosevly obout 12 hour after pitch. and fermenting for 3 weeks very slow in the end down to 1.015

now when its finnished it has a lot of body and tastes good but i get this oily feel in my mouth almoust like the same fell when you taste cream... slippery mouthfeel "lubricated"

why does it do this?

cheers

 

thanks yooper! I did not think of diacetyl at all, maybe because i cant seem to find any buttery butterscoth taste. i have reanalyse and se if i can find it afterall. i guess i could have eliminated tha diacetyl by boosting the fermentation temp at the end, which i didn´t

 

Are you still on the primary yeast cake? If you leave it on the cake for a bit longer it could clear up that problem, perhaps give it a swirl or two to get some yeast in suspension. Hopefully you just tasted your gravity reading (I always do that) and so are still in the primary.
I have cured a Diacetyl problem in the past by pitching a one liter starter into my lager and letting that chew up the Diacetyl. I resolved next time to just pitch way more yeast initially.