I have had the same problems lately. With all the potholes around I have punctured about 4 times (perviously not had one in a year).
Anyway I usually change tubes straight away and dont have any trouble replacing tyre and away we go.
The other day I was about 5 minutes (walking time) from work when I punctured. I thought sod it, i'll walk and fix the puncture later.
That turned out to be a bad move, I just could not replaced the tyre without pinch puncturing. The tyre seemed as though it had shrunk!
Got it done eventually, but I'll never leave one again it will be done immediately.