I posted this on my FB status...

I'm sorry that I didn't react well when you complimented me on my weight loss.
It's really 100% my fault. 
Please read my explanation.
Not too long ago, I had gained weight tremendously due to the side effects of medication. My illness was already too much of a burden and yet, people seemed to be more concern about my chubbiness.
I was hurt that the weight gain issue had literally outweighed my illness.
It was the worst of times.
So, whenever people bring up the subject about my weight, my thoughts automatically revert to that dark period of time.
My bad. I'm really sorry.
I know you meant well.
I'm not saying what I did was permissible, I just wanted my apology to come with an explanation.
From now onwards, I will smile and say thank you ... 
Just so you know, the weight I lost will find its way back to me eventually. (the medical statistics are strongly against my favour)
And when that happens, I will smile and thank you for your concern too..