With rentals there is no telling what sort of stance they gave you and they usually don't make any adjustment to it unless you ask, it's possible that either the angles were way different, or the stance itself was narrower; either one of those (or both!) could contribute to some trouble for a beginner.
Exactly what I thought. Since you got a shorter board your stance was probably more narrow and that will have a huge effect on how you ride. In general, beginners will want to be on softer boards that are a little shorter then they will ride once they have the basics down. All rental boards should be soft. Just make sure you are getting the same stance width and angles every time you go.