Why would your blood be no good? The only reason your blood would be considered bad is if you have HIV, hepatitis, be a man and have sex with men, or use blood thinners.

The reason why people who are skinny or underweight have trouble donating blood is because they do not have enough iron. I was 110 pounds at the time when I was first trying to donate blood and was rejected many times because I did not have enough iron and was only able to donate once at that weight. After I gained 40 pounds due to the meds, my iron is a lot higher. I don't know what your weight is, but have you ever tried donating blood and seeing how much iron you have?