nah its not underwatered. my first guess is that your soil has too much black dirt in it and not enough other stuff to air it up, like perlite, small wood chips, etc. The plant takes on traits of overwatering, but it could be the begining of a root problem.
the foilage looks pretty good, other than the color, thats what drives me to the roots.
I could be wrong, but if that's not it, then I don't know.