Place the stamp in the palm of your hand (face down) and hold it there for a few minutes. If the stamp curls up even more I would suspect regumming.
Don is giving you a good idead about possible re-gumming.
I have helped a couple people at out local stamp club who asked me to look at their gummed old stamp...They did not like it when I told them that their stamp was re-gummed...One quick way is to look directly at a perf..Real close..Remember stamp is gummed/printed and then perfed.
Perf machine removed gum in the perf hole to expose the actual stamp colour..If..If..You see gum down the actual perf hole..Probably re-gummed..
I did a very quick show of perfs..Original gum on left..Perf hole colour of stamp...Right perf hole has gum sliding down the perf hole covering up the original stamp colour..Not the best picture..Did it very quickly.