German Shepherds, especially cross breeds (along with Rottweilers) tend to have more frequent estrous cycles. Additonally, most cycles tend to last for about 21 to 25 days, meaning that a slightly longer duration of one cycle can shorten the time before the onset of the next. A German Shepherd breeder near us here in the U.S. says that the cycle tends to become more stable after the first pregnancy. He also states that if the female is near several other females, she may tend to come into heat each time they do (as do wolves).
He does say that if the periods continue to shorten, a Vet should check the progesterone levels as well as test for hypothroidism.
Finally, are you sure you're observing an actual cycle or are there symptoms you may be mistaking for heat? Some female dogs tend to lick themselves almost constantly as well as have slight swelling between heats. The telling factor will be, of course, the reaction of the neighborhood male dogs. Long way of saying I wouldn't be concerned about it at this stage...