Dog pooping and peeing outside the usual place

My dog is an indoor dog. I bring her out for walks in the evening at least every 2 days once and play with her in the house everyday. She has her own designated toilet where she does her business.

Lately, she has been doing her business outside of the toilet like in the hall more frequent. This normally happens also when i go out and come back and i see her poop and her pee in the hall. Further, i think she knows and realises this is wrong because when i open the door she immediately hides under the couch. She always does this when she knows she did something wrong.

My question is - is this a behavioral problem? Is she acting out? Is she being rebelious as normally 80% of the time she does her business in the toilet. She has been trained like that since she was a pup. What do I need to do?