The feeling of needles under your fingernails might be caused by insufficient oxygen supply or by local nerve damage. The two conditions are different, might not be related and might exist separately. They are also caused by different disorders. Insufficient oxygen supply might be caused by asthma which you mentioned you have. If the asthma is not regulated well, the oxygen supply through the whole body is not enough, but it might be most felt at the very ends of the extremities due to the local small blood vessels and capillaries. The skin under the nails might be the usual pink turning to blue more or less. Visiting a specialist for internal diseases to evaluate the severity of the asthma and perhaps change the medication is recommended.
Feeling of needles under fingernails might be related to vitamin B12 deficiency. In vitamin B12 deficiency, a demyelization occurs. Demyelization is a process in which the myelin (the layer surrounding the nerves and needed for their completely function) slowly decomposes. Vitamin B12 is a compound of the myelin and needed for its proper production. When demyelization occurs, the nerve starts to damage which might be first detected in the sensory nerves as paresthesia. Paresthesia is neural condition in which certain senses are detected without adequate stimuli. In vitamin B12 deficiency paresthesia might be manifested as feeling of pins and needles throughout the skin. The distant parts of the body (hands and feet) are more likely to be affected first. You might want to visit your family doctor and have laboratory tests to check if you are low on vitamin B12. He might want to refer you to a specialist for internal diseases to check if there is vitamin B152 malabsorption. if none is detected, the feeling of needles under the nails might solved by simply taking vitamin B12 supplements.