You can try the data driven approach. See if you notice a pattern to the "good" times. Could be linked to ovulation/hormones...or something you do is making her feel a certain way toward you.
And I agree with not being too tentative, self-conscious, or focused on her in the moment. Know what she likes best, and do that, certainly--but make sure you're conspicuously enjoying it for yourself because it's what YOU want, instead of trying obviously to please her.
Passion and desire unlock a lot, especially if they're a natural continuation of something that began outside the bedroom & not just an indication that you're horny.
This last is more risky, but...while too much scrutiny & checking in during sex is a turnoff, and while her being mellow doesn't necessarily mean she hates it, there's a limit to detachment in bed. At some point, a checked-out partner is a turnoff for YOU, and as one who feels the same, I see no reason to hide that.
If a woman I'm with seems very obviously "somewhere else" or preoccupied during sex, I stop & calmly suggest doing something else for a while, discussing her disengagement only if she asks why. You don't need to accept lackluster, bored sex, and that can actually work in your favor because it shows you are tuned in and care about her perceptions and fulfillment. It signals that you care about feeling connected, not just getting off.