User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-7418318-20160926124441/@comment-7418318-20160926203228

Oh I see!

This is Natsu's demonic puberty that awakens both his demonic powers and his... shall we say adult needs, desires and feelings.

And all the blood came from a massive nosebleed he got from seeing Lucy topless. ^^