Why have I been getting nauseous and sweaty before and during a bowel movement? The symptoms immediately go away afterwards and I feel fine. But I've been getting constipated lately, is it due to constipation or is it due to something else?

Could be anxiety at being constipated and then relief.

I have a bit of this as I get into my late 60s even if not constipated. How old are you?
I just assume that it has to do with the full bowel pressing on nerves. As we get older, organs sag, and that means more pressure on the lowest parts of the abdomen. I have to pee more, and seem to 'feel' more of what is going on lower down. I don't get sweaty, but do sometimes get a little sick feeling, and also my legs hurt a little. So I eat enough fiber in fresh vegetables and rice, and make bread with multi-grains. One baked apple does wonders for me.
Try 1 Colace every day as a very mild softener. It won't give you that sickening laxative feeling at all. Metamucil, if you don't mind drinking a very large glass of water with it.