Beagle always pees in the house?

My 3 and a half year old beagle is completely potty trained, since we got her when she was about one and a half, but lately she's been peeing in the house a lot. We always have the door to the backyard open throughout the day, except when everyone is out of the house, but before anyone leaves, we always let her out for a pee. Even if we leave for 5 minutes we come back to a huge puddle of pee. She never did this, and she never does it when someone leaves to go to work, or when I leave to go to school (during the school year of course). It has become a big problem, since the carpet is starting to look disgusting, and my dog is started to make this a habit. No one in the house wants this. She is a difficult dog, hard to train especially, for potty training. (I tried to get her to go in the same spot outside for about a month and half and she didn't listen whatsoever). We are going to get her vaccinations very very soon, so I will most definitely ask the vet about it. But for now, I just want to know, if anyone can tell me, why she does this? How I can get her to stop, since I am the one that trains her. Hoping you guys can be some help, thanks :)