Yeast's second wind..?

[CJHG_1]

So this has happened now almost every time I've brewed, including the current batch I have going - I'll pitch the yeast and I'll get very nice and very active fermentation that slows down in about 4 days. Then after about 10 days I start to see little bubbles that come up from the trub (I use a glass carboy). I wasn't able to know for sure if the fermentation was done (just got a refractometer a couple days ago) so to be safe I let it hang out for another week. Now a full 3 weeks later it is going WILD, like how it did in the first 3 days of fermenting. Ok now before someone says "CO2 release", I mentioned I just got a refractometer and I took a gravity reading 2 days ago which was 1.027 (uncorrected because I didn't have the OG but that shouldn't matter for my purposes) and today the reading is 1.024. The refractometer was freshly calibrated before each use. Is it possible that fermentation just randomly started up again? And why is this happening for my fermentations just about every time?

 

[Munster51]

Refractometers are good for wort, not so much for beer. With that being said, you should rely more on your hydrometer to check to see if it is done fermenting.
Are you using any type of temperature control or have you recently had a spike in temperature?

 

[CJHG_1]

I just have it in a closet at room temp (usually around 72-74 degrees, which I know is a bit warm..) and I don't think there has been a spike in temperature. The thing is this has happened just about every time so if it was a temperature spike it would have to spike in temperature in a similar way every time that I brew.

 

[Munster51]

As the yeast eats through the sugars, it increases the temperature inside the fermenter. So you could possible be 3-8 degrees warmer than the ambient temperature.