nikon - Two pictures with the same setting come out with different lighting

I am new to photography and use a Nikon D3200 to shoot jewelry in a light tent. I use the original 18-55mm f/3.5-5.6 lens with Xit XTETN Auto Focus Macro Extension Tube for macro shots.

I have this problem where I would be shooting perfect images under a certain setting, then all of a sudden the light drops even though nothing changed (none of the light, settings, objects changed). See below two pictures taken a few seconds apart; and I took a third picture with a slower shutter speed but the picture came out even darker. This happened to me a few times already and I can't figure out why. Any advice?

Answer

I ended up taking off the extension tube, or replacing the previous extension tube with another one with a longer lens, viola, problem resolved! I can only assume the problem lies with the extension tube. Go figure...