This ha confused me for some time. According to the text Infrith is standing behind Hretha. Around 9 PM her name is grayed out and the mouse-over says that she's not currently available. 2 hours later the text hasn't changed but she's now currently available.
It's possible after some scenes she needs about an hour to recover. Usually the missionary or breeding ones, but I can't be sure. If that's the case, she's there in the tent, but not available for fun until she rests a bit.