Re: Spit up
Stress can inhibit the letdown reflex, but there is no evidence that I've seen that it has any measurable impact on milk composition (at least not to the degree that you would observe any kind of effect on your baby). It's good to find ways to deal with stress for your own health, but I wouldn't worry about it affecting your milk.
Some babies just spit up a lot, and the amount of spit up you see on any given day can be quite variable. My son was a spitter. He would just spew all over everything and everyone all. the. time. I had a receiving blanket draped over me most of the time because a simple burp cloth was by no means even close to enough coverage to protect me from the spit up. If there is visible pain during the spit up then you should look into possible reflux, but otherwise I wouldn't worry about it. Also spit up is common if you have oversupply and/or overactive letdown.
Don't pump! Pumping just causes your body to make more milk which leads to more pumping which makes more milk and so it goes on and on. I used cold compresses for the pain. I've heard cabbage leaves work too. I also needed to massage the breast in the shower to work out plugs. Sometimes I could get some milk to spray out while I did this, which gave me a little relief. If the pain is very bad or you are having problems with plugged ducts, then you can hand express just enough milk to get comfortable again until the next feed. Give it some time, the engorgement will eventually go away.
K. Sophia - Mama to my little lactivore, the amazing Mr. X (11/10).