Out of the blue toe walking on one foot, with almost 2 year old

Our toddler has been walking for about 9 months. He is almost two. I have hardly ever seen him walk on his toes but yesterday he started walking in his toes on just one foot. This was after he was running around at day care all day, walking/running just fine.

He doesn't say there is an owie, he puts pressure on it fine. Ive inspected his foot, not bruising, no marks, no slivers. Ive pressed all around his foot (to see if maybe there is a deeper bruise), no reaction from him at all. He has never had any medical issues beyond the typical early child illnesses. It just all seems to strange to us why this all of a sudden has started. Was wondering if anyone else has had a similar experience?