Puppy prefers to hold it in than to use her pad. What do I do?

My 8 week old puppy was doing fine and using her pads when all of a sudden she stopped using them completely and would hold it in instead of using them. We bought her a big play pen and lined it with pads and at first she would use them and wouldn't hold it in but up until yesterday she absolutely refused to use them even after we waited for HOURS for her to use them she would just hold it in. And it wasn't that she just didn't need to go because as soon as we let her out of the pen she went ahead and peed right outside the door. We haven't taken her outside since she still hasn't had her shots and we don't want to risk getting her sick. Help!