Need help with a 120lb baby!?

Hello everyone. I am needing some advise here. I have a two year old, 120lb Neopolitan Mastiff. She is the sweetest dog ever, great with kids, and my 9 lb Mini Schnauzer. So here is the deal. Here lately, if I am putting her in her outside pen for a potty break, she will just fall down on our way to the pen. She isnt sick, actually she is in top health right now, its just she doesnt want to go to the pen. If I am taking her for a walk and turn in that direction she will just flop down and not move until I turn the oppisite way. It was cute at first, but do you know how difficult it is to get a 120lb dog off the ground? I dont always have the time, (sometimes it take 10 min. to get her to move) and its really frustrating. I dont have a fenced yard, as I have three acres and she cannot be chained. She will not potty while the leash is on, so what can I do to stop this behavior? The pen is huge and not more thann 100 feet from the house, and I never leave her alone out there for more than