Ugh, help me? Need advice?

My almost 4 month old puppy was doing amazing going outside to potty! And now he's not going it snowed this afternoon and he's so excited by the snow. He runs around the backyard.

But he isn't going potty he's been going inside.. he even went to the spot he usually goes sniffs around and runs around some more. I stood out there for a while. It's so discouraging.. I hope he gets it soon
It's pretty upsetting :(
Ugh, help me? Need advice?
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