Check on your 2015 Form 1040 line 61 (or Form 1040A line 38) to see if there is an amount for Health care:Individual responsibility. If so, you should be entitled to a refund of that amount if you had coverage all year through Medicare. You will need to amend your 2015 tax return and indicate you were covered all year.
https://ttlc.intuit.com/questions/3069046-file-amended-2015-return
