Caught in a Compromise Situation at Work

So there I was, just another day as the nurse for a local clinic. Got into an argument with my co-worker earlier over the scheduling of shifts, basically she wanted me to do all her work while she took off early and I couldn’t handle it anymore. We’ve had a thing going on since before this job began, a flirtation, you might say, but it’s always been strictly just talking and a little bit of play at after-hours drinks with some coworkers. Anyway, I’m running late today because my car broke down again. The clinic has a policy that anyone arriving 15 minutes late or more needs to be marked as absent but there were patients lining up outside the door by then. My co-worker Lisa was already here and when she saw me, we sort of locked eyes in this mutual understanding. You know how it goes, this unspoken tension builds until one day you just go for it. So I tell her about my car issues and ask if she could cover some extra time. She smiles that wicked smile again and suggests a compromise: the clinic is mostly empty today, so we can close early, have a few drinks, and then make up work at night when everything’s quiet. Of course there's always room for a bit more than just drinks. I'm not really sure how this works in our industry or with patients who might come back later and find things out, but she assures me it'll be fine, she's known for her discretion around office politics anyway. And hey, I’ve been wanting to take the next step for a while now. Today just felt like an opportunity that couldn't be missed. So here’s where my mind starts to wander a bit, I mean, we’re both adults and consensual about this, but is there something wrong with taking advantage of work circumstances when no one else is around? Is it really okay or am I going to regret crossing into territory that's supposed to stay professional? What do you think, internet, would anyone in the same situation see this as completely acceptable given the circumstances?