visual fermentation, three weeks into primary?

I brewed a recipe from Joy of Homebrewing, the Palace Bitter ale, three weeks ago. I used Safbrew S-33 dry yeast. OG was 1.050. This is an extract recipe, w/ 3 lb of Light Malt Extract Syrup and 2 lb of Light DME.
The fermentation took off over the first few days, and then died down. I then took off the blow-off tube and switched to an airlock. There was no visible activity in the airlock, but I wanted to give the yeast time to continue working, so I left it for three weeks.
Tonight I went to take a FG, hoping to bottle this weekend... I take off the shirt covering the carboy and there's a very thin krausen on top of the beer, and the airlock is -slowly- bubbling.
It seems like my yeast rose from the dead. The fermentation temps have been fairly consistent: I'm just keeping it in a room where the temp has been between 65 and 70 each day this winter.
The only thing I can think of is that somehow the temperature rose enough to reactivate it. It hasn't been shaken or anything.
My plan is to just let it keep doing its thing. Has anyone seen this or have any explanation why it's suddenly started some visible fermentation activity?

All other variables aside the only thing I could think of right off hand is that maybe there where some fairly long chain sugars in the extract that the yeast are just chewing away on. Did you take a FG reading? If so where did it stand?

BrewStooge: thanks for the suggestion. I'll take a FG reading tomorrow, I didn't want to disturb it because I was kind of surprised and wanted to figure out what was going on. I'll post the FG when I take it. Thanks again!