spice island,
It happens to all of us, and is a possible sign of too much thinking. I found it especially when I had a tough assignment - spins with that not-so-good student, for example, or an especially big cross wind day when things have to be just right to be ok.
Can I suggest a little refresher with an instructor you really trust (a tough one, where when he/she says you are ok, you truly believe it deep inside.)