I would guess muscle. It would be good if you look at bodyfat measurements, as then you'd have a better idea. Also, as you lose weight I believe you need to reduce your intake to continue losing at the same speed. Perhaps you're just starting to approach an actual RDI (but not there yet, still losing, which is good). My partner's weight did that. His graph is a slow curve from steepness to level.
I went from XL to now XS (and buying stuff in the Boys' sections). It's hard on the wallet.
I'm now at 59kg, which is where I want to sit (under 60kg actually, but 59kg gives me a buffer). We did our longest ride yesterday (60km/37mi), which felt pretty good. It hurt, but not enough to swear off doing it again.
Cheers,
Marc