Clementine has dropped 85 leaves in 5 hours - help!

So I've purchased a clementine tree for my mother's 60th (she already has a lemon which does well in her conservatory).
I've been keeping it in my conservatory for the last 2 weeks and it's been completely fine. I've made sure the door is open to keep the temperature even. But today it went from fine to losing 85+ leaves in about 5 hours and I just don't know what to do. Googling suggests I should water it more and not water it, keep it cool and keep it hot, put it outside and keep it out of the sun. I have no idea what to do.
Some points:
- The plant is about 2ft tall.
- It still has about 1/2 of its leaves.
- It has 3 large, yellow fruit growing on it.
- The surface of the soil seems dry to the touch, but it was watered from the base this morning.
I've been watering it with a small cup of water every 3 days or so, today I thought it looked a little dry and poured a larger cup of water than usual into the saucer, this is the only thing I can think that's different, but feeling the soil it feels dryish.
I grow all sorts of plants myself but I've never once kept a citrus tree, I'm completely at a loss. 

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Perhaps the move from the conditions in the shop to those in your house surprised them.
When I water mine, I tend to give them a good drenching and then leave them for a few days, but I don’t know if that is the “correct” thing to do.
I wouldn’t leave it in full sun but I would give it good light.
Sorry I can’t suggest anything more. Perhaps put the leaves that it has lost into a bag and give them and the tree to your mum as “a project”.😊
Pity you couldn’t have asked your mum, she seems to know what to do with citrus, but that would have given the game away! I am sure she will nurse it back to life, it will just look a bit sparse this year...
One thing I'm not sure on is, it has 3 large fruit growing on it (and several smaller, new fruit), would it help at all to remove any?
Hope you can get it back to good health soon!
Wisdom is not putting it in a fruit salad.
You asked about removing the fruit, if the fruits will still look ok when you are presenting your mum with it, leave them, if they are looking a bit ropey, cut them off. Since you have new growth already, probably won’t harm the plant either way.
Citrus do prefer their soil slightly on the acid side and can get yellowing leaves (chlorosis) with long-term watering with hard (alkaline) tap water, which is why Monty will use the more acidic rainwater, but he does have it in buckets where he is! Usually a citrus feed or a weak dose of sequestered iron will sort this out, but if the new growth looks healthy enough (it will be a paler green than the old leaves anyway), I would probably leave it alone until after the handover, by which time I am sure you will be mightily relieved!