While checking my cameras, I noticed this doe had a big bulge between her legs. I was thinking her fawn might have died, and her milk pouch is swollen.

Poor her! Do you intend to do anything to help her?

Not much I can do. If it actually is her milk pouch, there's nothing to do but wait for it to dry up on its own.

If it's a tumor or something similar, especially if it gets bigger, and I see her during season, I'll try to put her down.

looks like her udder is full of milk. Because of the angle of the picture it is hard to tell if she is pregnant. She is either in a late pregnancy or has recently given birth.

I agree with Oldtimr here

It's hard to say from that angle, but she may even be about to give birth to a fawn. However, it looks to be a tad low from the vagina area and may be a tumor or enlarged milk sack if she lost her fawn and isn't nursing one.