Help- weight loss or muscle loss??

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Hi so I'm 15 and in January I weighed around 75-80 kilos. I am a surfer and I row so weighing around that is pretty normal because of muscle.
Today, after stopping rowing for 3 months due to injuries I saw I weighed 60kg. 
I'm a little confused, because when my body looked it's best was when I was heavier rather than lighter?! Is this because fat is lighter than muscle so I've lost muscle and fat has taken its place? Please help