I spent a couple years trying BE (balanced embouchure), and when I first started out I experienced the same symptoms you're describing when playing above high C. They gradually went away over time.
I don't know what the science is behind this - hopefully someone can correct me if I'm wrong here - but I think we build up a tolerance to the air pressure. When I was doing BE, I went from having a range stuck below high C to suddenly playing high G's (when doing the exercises). I wasn't used to experiencing the higher air pressure needed to create those notes, hence the light-headedness. As Rowuk might say, I definitely had not earned those notes. Over the course of 6 months or so the symptoms went away completely. It could be that I built up a tolerance or become acclimated to the pressure, or maybe my technique just became more refined. I'm not sure.
If you were forced to take a year+ off the horn due to an injury, you might be able to regain the chops quickly but have lost that tolerance you built up as a result of gradually expanding your range higher and higher over many years.
My advice would be this:
1) Get a teacher, even if you only take a lesson once per month or less... It will do wonders for your aural concept and keep you on the right track
2) Ignore almost all the embouchure advice you read on internet forums. Keep in mind that very few people can play a loud double C. If you can do it without using a trick embouchure or compromising your sound and flexibility in the "money register", it's likely you're doing something right. That doesn't mean you don't have room to improve or that the advice of anyone who can't play a DHC is useless - just question everything you read and don't be too eager to believe everything everyone says.
2.5) Then again, that you injured yourself so badly it took a year to recover would suggest that there are some serious problems... In the first post you imply that you don't know how the injury happened, and later you say that it was the result of not enough upper lip and too much pressure, so you moved the mouthpiece up.... How do you know you won't injure yourself again? You're just gonna be more careful this time? Get a good teacher. Even if you only see them once a month it will go a long way. How much time and effort do you put into your playing? It sounds like you owe it to yourself to invest like $50-100 a month into your playing...
3) IMO, don't worry too much about this just yet. You've only been "back to normal" for a month or so? Be careful not to pass out when playing, and see if the symptoms lessen over the next month or two. If it doesn't get a lot better after ~3 months I would worry.