Took early retirement at 62. Now, once I turned 65, my IRS check lessened by $100. Why ?

I guess you mean SS check. You are now on Medicare and they deduct $100 for insurance to pay for prescriptions. You can quit that if you get another insurance deal, but only during "open enrollment" which is right now. So if you want to forego the prescription insurance, or get insurance someplace else, get to the SS office and make arrangements.