My guess too. Listen as you reel the film. You can hear the crinkling and feel a roughness when it happens. Sometimes you won't know it until you get near the end and it feels like you jumped a rail. 36 exposures should practically fill the reel. If you have two spirals left at the end, then you should suspect wrinkling.
It generally happens when you don't start right.
If in doubt, I prefer to unreel completely and try to make the film start better centered under the clip.