MIke, that's strange. When you set up your Pinterest account, you have the option of using your Facebook or Google+ or twitter account, or good ol' email. If you set it up using the info from anything other than email, for instance Facebook, you need to be logged in to Facebook before you can log in to Pinterest. Do you think this might be the problem?
I would suspect if you used of them (FB, G+ or Twitter ) and were not logged in to your chosen service. Pinterest would not recognize your login info and send you through account creation again. This is just a hunch.
If that is not the case, Pinterest's help site might be of assistance...
https://help.pinterest.com/en/articles/login-trouble
Hope this is helpful! Good luck! Pinterest is worth the effort!