What's the symbol that appears over my champions head for?

So I've noticed this a few times (I think it's when I'm in a game with TF - is it related to his ult) I'm not sure. Could be a random guess lol. Anyway, when I'm playing I'll sometimes see a symbol appear over my champion. I have no idea what it means. I've screenshotted it and the links below. http://postimg.org/image/tm8relatv/ Thanks
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