Chances are its simply got a leak somewhere if it was bad you would have seen coolant dripping, hopefully you'll be able to track down the leak and fix it, have a good poke nose with a decent torch....if it was internal problems you should have noticed running or temperature issues by now.

I had a leak last week, had to rinse the whole boxer engine down with the hose pipe to work out where it was coming from (small bore coolant pipe), sods law dictates the leak was underneath the intercooler which isn't the easiest to re-attach once removed, i re-fitted it and then found one of the tiny vaccum pipes had found its way underneath the blessed thing while i was messing about so had to remove it once again, doh.

The OP put in 2 litres of anti-freeze and then 1.5 litres of water!

Mk 6 Fiesta has a total capacity of 5 to 6 litres depending on model + petrol/diesel

As I read it, the OP initially put 2L of premix (i.e. already diluted 1:1 coolant) in

Then he put 2L of undiluted (ie concentrate, which I've never seen, but I understand requires dilution 1:1)

Then he put 1.5L of water in.

So its had 4.5L in total, and is a bit stronger than it needs to be.

Or I'm confused, or the OP is, but I think thats what he said.