Oh wow, that sounds MUCH bigger. I'd just ask the vet when you bring her in. Maybe she had a major surgery...maybe she hurt herself somehow. If it was an actual incision, versus cutting herself on something the vet will probably be able to determine that...if it is an incision though, there's probably no way to know why or what it was for unless more tests are done.
Our resident dog Bailey had a very tiny spay incision, but her's was done at a regular vet's office. Our foster was done at the local shelter, and her incision was (seemingly to me) much bigger. I just figured at the vet, they had my dog's history and could be more discrete, where as at the shelter, they're doing so many of those a day, they open them up so they can see absolutely everything they need to see while doing the spay.