and more pee. I need some guidance please

I have been pee pad training Bella and when she is in her ex-pen which contains her bed, food, water and pee pad at the other end she does perfect! My problem comes in when I have her out playing. If I'm holding her she is fine but when I let her down to play on the floor she will run around and play with her doggie sister (1 week older then her) Chloe and before you know it she is squatting and peeing. I can put the pee pad out in the living room with us even right under her nose but she will still squat and go on the living room carpet. I just don't know how to train her to always use the pee pads. With all my other dogs who i've trained to use the great outdoors I could stop them mid squat and bring them outside until they finished the job, praise them and all that good stuff. With her I stop her mid squat and put her on the pad and she does nothing. I can only hold her there for so long kwim. Any suggestions? I have 2 ex-pens and i've even tried to extend the area she has to both to make it more spacious and she went on the floor instead of the pee pad.. it was still in the same place it always is. I really really want to be able to spend more time with her running around in our living spaces but so far it just isn't happening. Oh and when I stop her mid squat I then have to chase her down and she hides under the coffee table thinking i'm playing with her.