Rising gravity or user error?

My double chocolate stout which is closing in on 4 weeks in the fermentor had a gravity of 1.016 at 72 degrees last time I checked it on 5/29 and just this evening I was all geared up to bottle since I was certain the beer would be closer to the FG of 1.012 and instead I found the gravity was up 4 points at 1.020 at 68 degrees. Odd no?

So I verified the hydrometer was accurate using some tap water and came in at 1.0 at 65 degrees. After verifying the hydrometer's accuracy I sealed the fermentor back up and put the airlock back in place and will give a few more days and see what I get.

Damn! That hydrometer sample was pretty ok for flat beer.

So I must have misread the hydrometer on 5/29 right? There's not some some phenomenon that causes a beer's gravity to rise after a certain number of days is there? I'm only 4 brews in at this point so I'm still learning the ropes.

1 of the readings was off. no way the reading can go up

Yeah, I figured as much as well but thought I'd ask just in case. Hopefully my first measurement was the accurate one but I don't see how it could have been.

Either way I'm anxious to get that beer conditioned and in my belly!