Retro charge for higher group prem when Medicare eligible

Can my husbands' previous small group health insurer retroactively charge me a higher premium because I am Medicare eligible but had opted out of Part B? I put on the original application (2/2008) that I was eligible for Medicare (I'm under 65, but on disability) but also wrote in that I was NOT covered by Medicare so there would be no confusion. We were covered by that policy until 4/2010, paying the premium they determined and billed us. But just yesterday, they sent the previous employer a bill for over $13,300 for retroactive premium on only me, with no explanation and total is due 11/30/10!! After making several phone calls to the insurer (Kaiser-Permanente) it appears it has something to do with my being eligible for Medicare but not opting for Part B. Isn't it too late to tell me that my premium is 3 times as much as they quoted and continued for over 2 years to charge me? Everything was disclosed on the application and did not ever change. How can they get away with this?