I've not know females to hold on to a plug where it would pop out when she was popped. One of two things come to mind, one, SHE is a HE and you just didn't get his peins to pop out and only a plug (which happens) or two, it was a bit of shed skin in the cloaca that resembled a plug. If you can, probe her or try to re-pop her in a few weeks to see if you get another 'plug' or see hemipenes.