Nikon d5100 producing extremely underexposed images

I have a most vexing problem.

I have a Nikon d5100 (roughly a year old) which up until two days ago handled exposure and metering just fine. However, at night, in a well lit room (florescent bulbs) in which I NEVER had problems capturing images at reasonable shutter speeds, apertures and ISOs (< 400), here’s what’s happening now:

1. In Manual, even at low shutter speeds (1/90 and lower) and relatively open apertures (3.5) – ISO 640 and less – my pictures are almost completely black. Pointing the camera at a bright lightbulb just barely captures the light with heavy shadows all around it.
2. Exposure compensation no longer has ANY effect on the exposure
3. Shooting in Aperture Priority mode at f/3.5 at anything lower than ISO 1000 reduces my shutter speeds to 1/2 or lower to compensate.

Yes, I know. ISO. Keep in mind, the room is quite brightly lit and I never had this issue prior to last night. That being said, the only way I can capture a reasonably exposed picture here is at ISOs 6400 and above. But even a picture at 1/125, f/5.6, ISO 6400 (manual) is still ridiculously dark.

I have taken the following step to troubleshoot, to no avail:

• Turned exposure compensation up to the max +5.0 – no effect
• Turned shutter speeds down to 1/40 and below – extremely limited effect
• Turned ISO up to 6400 and beyond – limited effect
• Update the camera’s firmware (1.01 > 1.02) – no effect
• Reset the camera using the buttons with the two green dots – no effect, multiple tries
• Messed with the metering modes – no effect
• Turned it off and on again – no effect
• Stared pointedly at the camera – camera didn’t even flinch

I am at an absolute loss here and I really, truly, need help.

Images:

ISO 100, f/3.8, 1/3

ISO 100, f/3.8, 1/90

ISO 1000, f/3.8, 1/90