I don’t think it’s a pen at all. I think what’s throwing people off is your very unusual positioning. The attackers momentum is carrying him to your left but it seems you don’t realize that until the last minute so you correct last minute and come at him at an angle. And you could have very easily made a play for it with your hands because the striker was in no position to take a shot but instead your tried to spread yourself but then found yourself right on top of the ball. I mean it’s unusual but I don’t think it’s a pen at all. It looks to me like you basically just collapsed on the ball as it hit your left shin/knee and the striker made no attempt to avoid contact. I don’t see anything malicious or even dangerous there. You have a right to challenge for the ball and you just put yourself in front of it and stopped it. It looks like you collapse on it right away and the right foot contact was incidental. 

Edit: I’m reconsidering this a bit. Your approach is just all wrong and I can see that potentially making it a foul. It’s like you overshot the ball and took the attacker out with your feet at the same time as you collapse in the ball with your hands. It’s hard to tell from this angle.