My red clementine tree is currently growing "like crazy" and even producing some flowers, but all fruits that were on the tree for a couple of months already are suddenly turning yellow and dropping off...

What could be the cause of this?
I have recently transplanted the tree into a larger container.

Picture please =)

Did 100% of them drop and were they all yellowed upon falling?

Will post a picture later today!

Yes, all fruits are yellowing and all (4! ) are dropping...

1. Over watering or under watering.

2. Too much fertilizer.

3. Bad soil.

Here are some pictures...

The whole plant:


Some of the new branches:


The last remaining (and largest) fruit, slowly turning yellow. Diameter of the fruit is 19mm:


For reference, this is a picture of the plant when I just bought it, beginning of April this year...


I use fertiliser with every watering. The amount of fertiliser used is calculated as described in this post...
link

Soil used is described here:
link

Too much watering could be something... we had a period with rather hot and very dry weather during the past weeks. I have watered the plant (and other citrus plants) whenever the young branches and leaves started drooping, which, in worst case, could have been daily, but usually every other day.

Nice looking tree you have there. I have one just like yours.
My tree dropped its fruits in the hot weather. It is better to shelter it these days so it gets only the morning sun. If you have a fast drying soil, combined with the high temperatures results fruits drop.

"Excessive drop may be due to drought stress, sudden high temperatures, low humidity, or nitrogen deficiency. Heavy pruning, thrips, mites, or spray injury can also cause fruit to drop. Keep trees in good health and well irrigated to minimize fruit drop. Fruit drop is a self-regulating mechanism in citrus trees."

Try to read THIS too.

Another Uploading.com link. I never see anything except their advertisements. - Millet (905-)

Looking at the small number of blooms on the tree when you received it, the tree might have held a few more fruit, but certainly not many more, especially for a young tree. A citrus tree always produces many more flowers then will ever become permanent fruit. Normally for citrus, only 1-2 percent of the flowers will ever produce fruit that will be retained on the tree until maturity, even under the best of conditions. Of the fruit that is originally set, a high percentage of them are discarded by the tree, in what is commonly called "June drop", whether they drop in June or not. Many times you can get a higher percentage of fruit to be retained by "pollinating" the blooms with GA3. - Millet (905-)

GA3 would be nice to try, but I haven't got a clue where to find that...

You can see THIS online without downloading it.
Millet, I am sure you know this guide.

It could be what Millet wrote also. But you said that you had some serious heat issues. And my clementines are the finickiest off all my citrus. I always have fruit related problems with my clementine, whenever it goes trough a shock (no matter what).