some dogs have irregular cycles, the weather or exposure to another one in heat can affect them. Have to agree though with the person who questioned you breeding, don’t! there are more than enough people out there breeding. A girl shouldn’t be breed before 2 years of age, when all health testing can be done, in gsd you especially want hips x-rayed and they can’t be certified non-dysplasic till 2 years of age.
gilrs can also have split heats, these happen if there is some trauma that causes them to not finish their full cycle ( like an injury- we might consider it minor but in their minds it isn’t a good time)

Cystic ovaries or a hormonal imbalance – she very well may not be fertile. If you are DETERMINED to breed, be prepared to lay out $$$ at a reproductive specialist vet. I’ve heard of this twice (both with GSD’s) in both cases – spaying was decided as the best option.

this is something that should be checked by a vet this is not normal.

Sorry to be rude. If you don’t know why this is don’t breed.

Dogs can go into false heats and can have irregular periods at the start of puberty. A call to your vet to confirm might be the best option – and calls to ask are normally free.