Im 5'7 21 years old and weigh 122lbs But when I reach 122lbs no matter what I eat or workout I start losing weight rapidly I just can't pass 122lbs whats wrong with me?

I believe that your body is telling you that it needs to be a 122lbs and if it drops any further your body will become unable to support you and result in making you slightly ill.

Instead of focusing on your weight, worry about your BMI or Body Mass Index. This statistic is a much better idea of how healthy you are because it takes your height and weight then roughly calculates your percentage or body fat. I can't remember off the top of my head what a healthy BMI is but i now the number is higher for females than it is for males.

I hope this helps

Zilduli