Once Sanji woke up after almost dying from nosebleeding he received blood from 2 individuals. He himself stated that he would feel off back then, what if this was an foreshadowing of him awakening his Germa Powers? It was when 2 okamas gave blood to sanji after he passed out. Just think about it, the Okama blood might have triggered Sanjis awakening. 
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Its just cruel fate that Sanji has to bear in order to come along with the new powers. 