but I just want to make sure.

If I make PR's and thus my muscles are growing and the scale says I'm loosing weight.. that must be all fat (and maybe some water) right?

sound right to me. check your bodyfat levels and compare from time to time for certainty

I think you are right

If you are losing weight but you think you are gaining muscle mass ,then I think the decrease is due to loss of fat and water only!

It is not necessarily ALL fat, but that's probably most of it. If you're not eating enough, the body can actually break down muscle to fuel itself, though this is rare.

Just keep in mind that it is basically impossible to develop strength while losing weight as quickly as you would if you were gaining weight. If you're losing weight it usually means you have a caloric deficit (energy expended > energy intake). This means your body is deprived of energy it could be using to build your muscles during your recovery phase.

But it all depends on what your fitness goals are, anyways.