carbonation

i was ready to bottle an american lager today, but when i took a gravity reading i tasted the sample and it had a head on it and was carbonated. it tastes great but can't figure out why and am hesitant to bottle before i know why. any suggestions?

it cant be carbonated unless it was under pressure. more likely it is still fermenting. Take a gravity reading every couple of days until you get the same gravity and the bubbles subside. If you bottle early you will have bottle bombs. How long have you been fermenting it?

it's been fermenting 7 wks. og was 1.060. fg was 1.017.  poured it from my test flask to a small glass. it had a nice head and tiny bubbles like champagne. it had a carbonation bite, like a beer that hasn't quite been conditioned, but pretty much ready to drink. i've brewed several beers over the years and never had this happen. the airlock was not blocked. 

you got me with that one, its a miracle lol. never heard of anything like that , are ya gonna bottle it as usual?

no, but i can draw a glass or two out from time to time. it's the first beer i've tasted in the carboy thats good enough to drink now. 

try bottling one and capping it and see what its like in a week

went to a local winery. the vintar said you can get partial carbonation without pressure. he said that he notices a lot of carbonization in his primaries, before outgassing and adding stabilizers.  he suggested twisting the carboy on a flat surface and twisting back a forth quickly. so i did this for three days and no more outgassing. the first few times  it had massive bubbles coming up through the airlock. i tasted , no more carbonation feel, and no tiny bubbles coming up to the surface. i am bottling today. i appreciate your time in responding. happy brewing!

I too have noticed a slight amount of carbonation from time to time from fermentation.  As for the bottling, the gravity reading you took sounds high for a logger.  I would maybe raise the temp some and allow the yeast to finish off and clean everything up.  It is pretty common for brewers to bring loggers up to low to mid 60 range for the tail end of fermentation.  This is important to assist in driving off the diacetyl that can come during fermentation.