Nate,
I don't know what is causing you to feel really weird and unreal (it could very well be anxiety), but I very much doubt it is due to a brain tumor. You should go to a doctor and discuss it with them. If nothing else talking with a doctor (if you have one that listens and you like) might ease your mind until the feeling of unreality passes and you start feeling yourself once again. I once had something similar happen to me when I was 16 (long time ago). I felt really bizarre, as if I was not real and everything around me was unreal and I was disconnected from all things. I felt as though it would never end, but it only lasted for about 2 or 3 days. I never knew what caused it and actually never told anyone about it. I had something similar happen to me again about 10 years later after undergoing general anesthesia-woke up feeling the same way, as though I was in a fog, very weird, disconnected from things around me. I knew this time it was from the anesthesia, but why it happened I do not know. It lasted about three weeks and then thankfully wore off. There have been other people on this board who have described similar events (some of the posts might still be on), but unfortunately they couldn't really shed any light on why it was happening to them either. I hope it is just anxiety and you start feeling your old self once again. Try to get a message in to the doctor if you can. If there is a psych forum in MedHelp you might want to try posting there as well.
I forgot that you mentioned headaches and loss of balance when standing up in your post. You should go to a doctor. There might be a connection between your feelings of unreality/weirdness and the headaches. I don't know if migraines can cause feelings of unreality or not. Maybe somebody who knows more about them can shed some light or you could do some research on your own. Good luck.