Why has my dog started to pooh and wee in the house when shes now 3 years old and was fully trained?

shes a 3 year old staffi, who gets a walk everysingle day. recently shes started weeing on the seat and its like she doesn't even know that shes done it. today however shes started pooing in the kitchen without barking to tell us she needs the toilet, shes done it three times today while i have been in and out of the house. i have shouted and told her shes done wrong but i dont know whether to still take her walks when shes been and i don't know whats causing her to make a mess, any suggestions please because im scared something may be wrong with her...