A natural 20 would tell you that Eren is putting some pieces together from your time on the road. He remembers you said that the man you're tracking is a "pagan" and yet your seeming affiliation with the old gods would also put you in the pagan camp. Now that he's seen you kill, he's wondering just how dangerous you might be as his unfamiliarity with the old gods makes you feel somewhat unpredictable to him.