It's not unheard off but it's quite unusual. Could she have been in season without you realising? My parents' collie showed no signs of being in season but as regular as clockwork, every six months they would be besieged by stray dogs barking at the gate.
I think thyroid issues can cause hormone problems. It might be worth checking with the vet.
I have a springer/lab bitch who has just turned 1. She's healthy, happy and generally crazy however she's yet to have her first season. Our vet advised us to let her have one before getting her neutered but i'm now getting a bit concerned. Anyone had experience of similar, not sure if I should take her to the vets or not. Shes definitely not pregnant so thats not a worry.