Hi there -
Almost certainly fluid overload was the primary issue. The question remains why this happened. Heart issues would seem to have been ruled out (congestive
heart failure is the most common cause of this sort of reaction, but apparently there's no indication that would be the case). You could also have developed a
pneumonia, but while there would seem to be no infection,
pneumonia is a catch-all term for inflammation of the lung with congestion due to fluid and/or mucus. Again, something else is usually at the bottom of it.
The treatment seems to have been appropriate and you've responded well, but I can understand your concern as to what may have happened. Fluid overload can, sometimes, be related to an undetected lowered renal capicity (kidney failure = fluid overload). You may simply have smaller than normal kidneys for your size, or there may be some other reason your body didn't eliminate the fluids as expected.
There is also the remote possibilty that you received more fluids than were appropriate, but this is unusual and would be difficult to prove. The good news is that the situation has been corrected. By all means, should you ever need to have surgery in the future, let the doctors know this happened once, so they'll be on the lookout for it, whatever the cause. It doesn't appear you have an ongoing problem with this, and with the high blood pressure you experienced, it really does seem you were just flooded (fluid overload). I'm glad everything turned out well. This can happen to young, otherwise healthy people, but it's rare unless too much fluid is administered or the output isn't followed adequately. I strongly suspect the latter was part of the problem.
I hope this helps some. Best of luck to you.