Tanita is fine as a scale, but notoriously inaccurate when it comes to body composition. And BMI = body mass index, which is calculated mathematically, only factoring in your height and weight, so if you've lost 34 lbs, your BMI HAS changed...your scale just can't do math.
At 217, your BMI would have been 37.2, and now at 183, it's 31.4. If you want to calculate it yourself, the formula is: weight in lbs / (height in inches x height in inches) x 703, so yours would be 183 divided by 4096 (64"x64") = .0446, times 703 = 31.4. Or here's a link to an online BMI calculator:
http://www.nhlbisupport.com/bmi/