Pretty much one of your largest veins gets stuck/compressed between your kidney and another vein. It can range from a large part of your vein being stuck, or just a small part being pinched. It can put pressure on your stomach when sitting, standing, walking, etc. Causing you to throw up everything you eat and drink from pain. It also gives you back pain and pain under your left lower ribs. Also it causes the blood to have to reroute and find another way to get to your heart, which it always does but this can cause your heart to randomly beat faster at random times, or in my case chest pain that can last up to a couple hours.