Sometimes they deflate when they expell waste
BUT...how old is your tank? And what type of lighting do you have?
How long have you had anemone? What do you feed it?
My bta did that when I first got it. Now it does it less often ... i guess it just needed to get comfortable. I have the same lighting as you in a 65 gallon and i've had my bta for over 6 months now and it's doing well ... eating squid and shrimp.