Hey guys. I am sorry if this has been asked myriads of times, but I did a thorough search and could not find a thread on the exact matter, so I hope the admins will bear with me if I missed something. :)

I recently received a young Lemon tree (bearing some fruit) in a pot as a housewarming gift from my in-laws. It had been in the care of a relative of my girlfriend, and the relative had not been in possession of a proper fertilizer.

So when I got it, it was in pretty bad shape. I bought some citrus fertilizer, was careful not to screw up the solution, and did my best not to sog the roots.

Over the next few days, the state of the tree improved, and one of the fruits ripened at breakneck speed - only to drop from the branch quite suddenly.

And seeing how I read something about lime in tapwater being bad for lemons, I got a bit nervous that i might have done something exorbitantly wrong.
It has to be said, though, that apart from the random drop, it is setting new leaves and looking quite decent.

So my question is.
Is it a result of stress, overwatering, or?

Thank you in advance :)