Why would I throw up for no reason?

 Why would I throw up for no reason? The last food I ate was 4 hours ago. It was normal food that I have eaten often (PB on some bread). I felt like I was going to throw up for 30 mins then I threw up nothing but some gastric juices (stomach was empty). Was awful.

My guess is that you did not really through up, but you experienced the dreaded "foamies".  That is when all that comes up is gastric juice.  It is really uncomfortable, but it will pass.   This happens when we eat too much food, or we eat too fast, or we are not yet ready for the food we are eating.  It sounds like you ate too much or too fast.  Slow it down and listen for the full signal.  Sometimes we miss that signal and pay for it.  It happened to me maybe six times in all.  The further out I get, the less likely it is to happen.

This can happen if you have a piece of food that gets stuck at the narrow curve of the stomach at the top.  Then you start backing up all the saliva that you are swallowing because the food makes a plug.  The plugging happens when you either eat enough to completely fill your sleeve to the brim or you have a piece of food that is stuck in the curved portion of your sleeve at the top because it's too big to pass.