It is normal for anenomes to do this. Chondys do it dramatically, they can shrink up to 1/3 of thair actual size.
They do this to release water and flush themselves out. It's their way of removing the waste left over from eating krill or brine shrimp 2 days before.
If you suspect your anenome is sick, i recommend increasing the light around it and relocating it to a secure part of the tank. But once an anenome starts to die, they rarely recover as ther motabolism is extremely low.
HTH.
btw,
tell us about the tank and lighting, are your levels all good? trates/trites?