Our expert says:
It could be muscle, yes. Certainly part of it would be. Given that you started on the underweight side, the normal response to training may well be to increase mass. There's also an increase in plasma volume (literally, you make more blood) which would help.
Then, when people exercise, they do sometimes increase their energy intake, because exercise has the effect of making us hungry. So it's possible that you're making tiny changes to diet that are causing this. That's something you can only really figure out if you keep a food diary (which is a mission!) but it's possible, so be aware of it.
I think the key is that you're still nowhere near overweight, in fact you're still quite well underweight, and so as long as you're not just adding fat (and you'd see this), then keep going, because you're going in the right direction!
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