Too ticklish? I just talked to my ladyfriend on the phone a few moments ago. I gave her a tickle session last night... about two and a half hours worth of tickling. Now, I do the light, gentle feathery tickling. She just told me that what I did last night was way too overstimulating and that it almost felt more painful than pleasurable. I always mix it up a bit... tickling here and deeper massage there. Anyway, she said she almost felt like calling off the session but since she knew that I enjoy tickling her, she'd let me continue. So, in answer to the main question... apparently, yes, it is very possible to be too ticklish.
BUT... this very same lady has told me on numerous occasions that her body craves my light tickling. I guess the skin's sensitivity changes like the wind. Hers seems to, anyway.