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Ive been having this problem every once and a while. Could some explain to me whats happening

Are you using "Matrix" metering?
Are you shooting Jpeg or RAW?
What shooting mode are you in ?(Ex: Aperture, Shutter, Programmable, Manual)

If it is an auto mode, It is probable due to the EV compensation (if any) and the metering mode.

I would recomend "Centre weighted" metering.

HTH

There could be several reasons. Using a flash or not, what metering setting and what mode? One of the first ideas that came to my mind was matrix metering in a programmed or auto mode and when the door was opened it changed the setting.

Well, clearly the shot is under-exposed. Probably had something to do with the bright light in the background and white and reflective foil decorations and such all coming together and conspiring to make the camera's internal meter think the shot was brighter than it actually appears to the human eye. In response, it decreased the overall exposure.

Remember it's not the job of the light meter to calculate the exposure you want, its job is to give you a consistent base-line of exposure (based on middle grey) which you use to calculate the exposure you want. Looking at the histogram would have shown you how to correct this. If you took that shot that in raw, I'm sure it would correct easily. Possibly in JPG as well, but obviously not as cleanly.

I shot raw

By the sound of the camera it sounds like the shutter isn't opening all the way

What re you using for processing? IF Lightroom then bring up the Shadow slider.

I use Lightroom 6

jayhunter said:

I use Lightroom 6

Click to expand...

Under the Basic Tab bring up the Shadow slider, raise the exposure and in general clean it up. Only had the small jpeg to work with but you get the idea

SimilarView attachment 213493

jayhunter said:

By the sound of the camera it sounds like the shutter isn't opening all the way

Click to expand...

AFAIK the shutter in a DSLR moves downward at a rate based on the shutter speed.
The mirror is respnsible for ensureing that the sensor is not exposed to light before it is time to, Except if there is a electronic front curtain.

As has been pointed out, This should be easily fixed in PP.
You will probable have more noise in it due to the under exposure.

HTH

All we know is that its a D3200 and the implication is it does not "sound right"

flash ? Mode ? etc etc