1099-SA for HSA increased my taxes despite Only used for Medical Expenses

I just turned 65 this past March and am now on Medicare but I am using my HSA from my employer to pay my medical bills. 

I put into Turbo Tax my distributions and it raised my Federal Taxes by A LOT, even though I checked 'only used for medical expenses'??????? 

Is there something special about turning 65 that made it suddenly taxable? The whole questions about 'do i have a HDHP currently threw me off as i have medicare now and don't contribute to my HSA anymore, it is just sitting there in an account to spend. I must be missing something?