Some of my best sessions with my T I have a hard time remembering the specifics. And actually, I find that the more you don't remember, the more it was helpful/enjoyable. It means you're more in the moment IMO... and sometimes a feeling, you just can't put words to. Sometimes, a feeling is more important than any explicit "cognitive process"... it gets at something deep inside of you - and that can be very difficult to describe, or even think objectively about afterwards.
Of course, this sometimes isn't very good, as you tend to want to remember every little detail about those sessions that go well. You want to hold tight to them so you never forget ... but as the saying goes, the harder you try to hold onto something, the faster you lose it (or something along those lines!)
As for telling your T about it .. say what you said here? Say you're not sure of the specifics of what was especially helpful, but just that she helped to bring you into the present, and "relax from it's scrunched-up worried state." There's nothing wrong with that!

Jacq