I wouldn't be concerned about her weight at all, pp have explained why. DS is in the 5-10% percentile. He gets called everything from 'tiny' to 'normal' to 'gee he's big, isn't he?' (NO, he's not, but thanks for asking!!)
But, I agree with n2l that the worrying part (sorry to use that word, I know it's not going to help you feel better) is her weight loss. If she's been sick, it could explain it. Could the force feeding could be turning her off food, making it feel like a punishment instead of something fun, normal and natural. Does she have a favourite food? I thought the 'adding secret ingredients to her food to boost it' was a great suggestion! Wait and see what her test results turn up, and try not to worry too much in the meantime (easier said than done I know). You're doing a great job!