I'm inside a lit room with the shutter speed at 1/125s, aperture is f/5.6 and ISO is 400, but every photo I take is still really underexposed. Even if I have the shutter speed on 1/60s and I hold the camera up to the light, the exposure is fine so this doesn't make any sense.
i'm inside a lit room with the shutter speed being 1/125, aperture is 5.6 and ISO is 400 but every photo i take is still really under-exposed.
If you take a picture with these settings, and if it's not underexposed, that's a helluva amount of light in your "lit room"! I have nearly 400 watts of LED lights producing over 40 000 lumens on the ceiling of my house. Yet, I looked at the pictures I have taken indoors and I have one where the aperture is 5.6 and ISO is 400.
Guess what the shutter speed is?
No, it's not 1/125.
No, it's also not 1/60.
It is 1/10!
I suspect your "lit room" doesn't have as much lights as my lit room. Lit rooms are surprisingly dark, actually. You don't see it as dark with your eyes because your eyes can adapt.
Answer: crank up the ISO. A lot. Something like ISO 6400 might be a good start with f/5.6 and 1/125 shutter speed. Alternatively, you can open up the aperture if the lens supports it, or use longer shutter speeds (preferably with image stabilization, if your lens has an image stabilizer), because ISO 6400 might lead to too much noise.
i'm inside a lit room with the shutter speed being 1/125, aperture is 5.6 and ISO is 400 but every photo i take is still really under-exposed. i [mean,] even if the shutter speed is on 1/60 and i hold the camera up to the light, the exposure is fine so this doesn't make any sense.
You hold the camera up to the light and expose with 1/60 s exposure time. You expect that to be similar to the result when photographing the room with 1/125 s exposure time. That sounds like you are adjusting in the wrong direction. 1/125 s exposure time delivers half the amount of light from 1/60 s exposure time (exposing half as long), so if the light is well exposed using 1/60 s, the room would have a chance to be well exposed using 1/30 s or even longer.
Holding the camera up to the light is not a good way to check exposure or ambient lighting.
Increase your ISO to 800 or 1600 or even 3200 and see if that solves your problem. If it does, then it is just a very dark environment.
If it is still underexposed, your equipment is faulty.