In my opinion, moving her to to the portacot has worked against you. She will be getting mixed messages. If you make the move to a big bed, then I'd stick it out - the not sleeping and getting out is quite normal, you have to teach them the new boundaries.
Its awesome she sleeps through the night, well done - thats usually the hard bit.
Do you put her down for a nap just after lunch? Even if she knows this is 'rest time' - regardless of whether she sleeps, it will help. There are a few things you could try. One is shutting the door and just letting her fall asleep where-ever. Another (which we used) is to discipline. As soon as she gets up, put her back in - be quick, firm and down to business. Its a battle of the wills, so the stronger you are and consistent with your approach - the more likely she will realise these are the boundaries and simply comply.
Anotehr alternative is she just isn't needing a day nap any-more which is common. You could try moving her night time sleep forward or even later and see if that changes any-thing.
Good luck, its a hard transition at this age.
DD is 3yr 8 months - DS is 6 months