@Hollyj, take deep breath and engage with THS.
I mentioned this in another thread but we’ve had two sit cancellations. Each very short notice. One California. One Southern Europe. For us, a key factor in outcome was the presence (or not) of alternative housesits. Many in CA, none in EU. We rapidly secured alternative housesit in CA (Sit Cancellation Insurance probably ineligible). We used Sit Cancellation Insurance in EU.
You note Pet Parent indicated stay extra night. That may permit you to regroup and make an action plan. Subjective but arguably poor character that they offered you the couch (they could sleep on couch or with friend/family for one night). Perhaps good reason.
I can’t comment on timing of upgrade to Premium membership. Fingers crossed that THS are kind to you.
Well done on contacting THS. They will provide policy info and guidance. Recommend read carefully.
Thinking of you. Hope works out. Once you have a new action plan, it’ll feel much better. Keep smiling 
That sounds incredibly stressful, and it’s a good idea to get to the root of this abrupt decision. Asking if something specific concerned him, especially regarding his pets or the home, could help clarify if there’s been a misunderstanding or misinterpretation. Maybe there was a hidden camera involved? Maybe he saw something and was upset about it? Not saying that you have done something wrong, I’m just interested if he might have misinterpreted something. Bringing up this directly and professionally could also signal to him that his sudden change has left you in a difficult situation, especially given the distance and full-time nature of your house-sitting.
If there was a hidden camera involved, it’s of course not only unsettling but also against the platform policies.
Miight this be a stumbling block, since you initiated the cover after being told the pet parent was thinking of coming home early? Have you received any advice from MS on this @Hollyj ?
Yes, that does sound strange. Unless the roommate travelled with him and they’re both returning home early? Whatever the circumstances, it’s not good for you. I hope you can get things sorted.
Edited to ask: will you be staying until the host returns, or departing before he shows up?
That was my first thought, I’m afraid, me being old and cynical. Surely if you’re sick, you mention what with… “I have an upset stomach” “Looks like I have flu/COVID/beriberi…” “I broke my arm” or whatever. But just “I don’t feel well” is… Odd. Then when you mention the roommate… Nah. This is not straight-up.
I had a sit, in 2 weeks, cancel. But I was lucky, since I was staying with a friend, the night before the sit started, so just staying there for a few days, then heading to family for thanksgiving. Alternatively, I have a sit in Albuquerque in mid Dec that suddenly switched dates, I have a sit that overlaps before, so they are going to make arrangements until I get there. Any last minute changes are stressful when you are on the road and trying to figure out logistics. I do have financial back ups, but still a pain.