My thoughts:
Anaemia is a reduction in Haemoglobin within the blood (part of the red blood cell - there are 4 Haem groups per red blood cell).
However, most well people have pretty much fully saturated Haemoglobin in normal room air (ie 21% oxygen). Check this out by putting on a Sats probe on your finger. So breathing 80% nitrox is not going to make much difference. The only difference it will make is in the dissolved oxygen within the blood, but think this is going to be a relatively small effect. The time higher oxygen concentrations are used is those who have chronic lung conditions where the effectiveness of the lung is greatly reduced such as those with Chronic Obstructive Pulmonary Disease.
Rather you need to look for the cause of the anaemia.
Aching and painful muscles is as a result of anaerobic respiration. Due to not enough oxygen they respire anaerobically and hence lactic acid is formed as a by product. This is what gives the ache and pain.
Hope that helps