Hi Amy. All the advice above sounds spot on. I have a four day old but this is my second- unfortunately I am again dealing with sore nipples. I have no advice on the runny poo, but don't forget that alternating panadol and nurofen can help at least a little with the nipple pain. I can totally relate to the crying during feeding from my first and the pain can be overwhelming, especially if you are still dealing physically with a difficult birth.
Be kind to yourself and do whatever you need to do to get through it. Best of luck.