Posted: 19/11/2015 at 09:17
How big is the tree relative to the pot? Lemons (well, their root structure) can get lost in pots that are too big for them and will suffer for it. I don't mean they should be pot-bound, just that the root structure is restricted to an extent. As the tree grows, and the root structure develops, they can be potted up, but always with an eye to restricting the roots.
It's common for leaves to yellow and fall off. Sometimes it just happens, or it could be overwatering.