Hoping you can tell me, is cancer diagnosis shortly after beginning health ins coverage a preexisting condition?

Probably not: This is a tough question and is debated amongst the insurers. Technically, the cancer was probably there before beginning the health insurance as most cancers don't "blossom" overnight. However, if there were no signs and symptoms and no prior diagnosis of it, then it would not be a pre-existing condition. This is often the case when a cancer is found on routine screening tests (ie. Mammagrophy).