Probably just some excess water but muscle loss is to be expected, so it is most likely a combination of the two. Were you taking any maintenance dose AI's on cycle ? Also did you do the hcg supplementation as suggest in the pct examples ? Hcg also will increase estrogen levels significantly so the increased water retention can easily cloud the muscle loss experienced post cycle.
Anyways, 2kg is not much. What were your gains, cycle length etc etc ? All seems normal, glad your happy with your results