Omg this is crazy helpppp

We have a Pom/shih tzu she is a little over three months. She is so smart, but this potty training has us so confused. We are retired and have lots of time to give her. She’s has a pen and she loves it. She sleeps all night in her pen with no accidents. We take her out to potty about 5 and she will go back to sleep and then get up about 6:30 or 7. We take her out constantly on a leash out the same door to the same place on the front lawn. She pees almost as soon as we get on the grass and is always given a treat and lots of love and praise. She had like three accidents in the house today. So this afternoon I set an alarm and took her out every 15 minutes she pees every time we take her out. She always poops outside. She doesn’t tell us yet if she needs to go out. I took her out every 15 minutes for like two hours. The last time I took her out she peed right away. We came in and I was playing with her with her ball. She has a small puppy bed that is not in her pen just at the end of the couch. Two minutes after we came in I threw the ball and it landed in her bed she ran to get the ball and jumped in her bed and squatted and peed in her bed. Then two minutes later she squatted and peed on the floor each time was a good size puddle. We had her tested to make sure she doesn’t have a uti and it was negative. How can she have that much pee in her that she can pee three times in just a couple minutes. We will never be able to potty train her. She will have to be in her pen all the time 😢 because if she’s allowed to walk around the living area she constantly pees. We are to old for this lol help please