She offered to put you up so I would try to accept her offer at face value and not fret about it. She should not have mentioned the thing about "putting off" Valentine's Day with her boyfriend--if she had just kept silent about that, you would never have started to worry you were intruding.
I would just do whatever you would normally do, if you were staying a night with a friend and it was March or June or something. And that might include saying something like, "So, were you thinking that we would all have dinner together that night, or should I go out for my own meal, or what? What's the plan?"
I guess when I think about a couple have Valentine's Day plans, the first thing that comes to mind is romance/Scrabble. I feel like that would probably be inhibited by a guest no matter what--if they're in the next room, if they're out but we have to be alert for when she comes home, etc.. It might seem nice to tell them to just go ahead and follow their plans, and you'll stay out of their way, but logistically that might actually not be possible, and they'll end up with a watered down celebration when they could just wait until the next night when you're no longer there. Your friend offered to let you stay on that date and said it was fine, so I wouldn't bring up what they might be putting off as it seems likely to cause awkwardness.