Reeemap - Have you been diagnosed with diabetes? What sort of numbers are your blood sugars running at? Over what period is your weight loss?
How are you feeling? Are you otherwise well; not drinking loads, weeing lots?
Of course, it is usual to gain a few pounds whilst pregnant, and ideally that be lost afterwards, folks saying you look thin could just be that they've become accustomed to you carrying a few more pounds, and if they haven't seen you for a while, 18kg is quite a bit to have lost.