When they shrink it's primarily a response to stimulus. This will either be an irritant, change in light intensity (on/off) and/or feeding. More often than not feeding will result in the anemone shrinking when it expells waste. As long as it's color is good, the foot is firmly attached and the mouth is normally closed, it should be fine. Try not to feed it too often though. BTA's will respond better than most species to direct feedings but you can easily overdo it.