My understanding is that this can be quite normal behaviour. They can just float a the top for periods of time with the arms and legs stretched out. They are just relaxing/being lazy.
There could be other reasons, however, such as bloating --which can be food related or, sadly, disease-related. Does it look okay physically? Things to look for are the eyes (should be clear and not puffy), reddish-pink patches on skin, white fuzzy patches, etc. Is it eating normally? Has it experienced weight loss?