Something badly wrong with my dog?

Okay so I have 2 Shih Tzu's one is 4 and one is 5, and throughout all their lifes they have never had any behavioral issues. However lately, in the space of literally a day, The slightly older dog has totally changed, but only at night, during the day she's pretty much normal but as soon as it hit nights, she starts panting, all night, and then scratching the back door that leads to the garden, all night, for hours and hours on end, I've tried positive and negative reinforcement Ive tried ignoring it, nothing works and it is generally driving me crazy, At night she doesn't listen to anything I say, For example tonight she was doing it and I absolutely screamed at her, as loud as I could, the younger shih tzu ran to his bed (I shouted bed) and the older one just tried to get past me to get back to the door so It could keep scratching, Even now she is still panting but it's only at night,

Please, can anyone help me, I'm really worried,

thanks !

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