I want to get a better mobility and flexibility. Therefore, I am going to start stretching several times a week. However, I don't want to get super skinny because of it. Does the stretching have negative influences on building muscles? Will I lose muscles because of too much stretching? I am a little afraid that could happen. Is my fear justified or am I worrying too much?
Stretching is actually crucial for improving the health of the fascia tissue. And this will actually improve your muscle building. I'll explain you why.
First of all you have to understand that hypetrophy occurs by two different stimuli, and one of that is a mechanical hypertrophic stimuli: if you want the full explaination of hypertrophy you can find it in my answer to this question right here.
Mechanical tension is distribuited throughout the muscle by the cytoskeleton, and this structure is thightly linked to the fascia/connective tissue of the muscle cell itself. When you stretch you actually improve the health of this tissue by increasing the turnover in the collagen proteins and modifying the structure itself of the tissue texture ( that will get what is called a "CRIMP" structure ).
If the transmission of kinetic force is optimal, you will not only have a whole bunch of benefits in terms of injury prevention, and performance, but you will also allow the tension to be transmitted to Z discs and basal lamina of the outher sarcolemma that are know to contain mechanotransductor proteins such as MAPK.
These proteins translate the mechanical stimuli in actually protein synthesis signals and will improve hypetrophy expecially of the sarcomere and myofibril structure.
This will happen only if you have a good transmisson of transversal force by costameres of the cytoskeleton and a healty CRIMP-like fascia to support it.
On top of that take in consideration that mechanical stimuli ( eccentric phase ) is also fundamental in the paracrine secretion of IL-6 by macrophages. Interleukine 6 is a potent stimulator of satellite muscle cells differentiation, and this is crucial for the repair and the growth of your muscles.
You want to optimize passive and active force transmission with a healthy fascia, and you can do this through stretching.
Does the stretching have negative influences on building muscles?
A stretched muscle is longer, and is therefore looser. This can increase the difficulty of producing a stretch reflex (in the squat [PDF], for example).
Will I lose muscles because of too much stretching? I am a little afraid that could happen. Is my fear justified or am I worrying too much?
You will not lose muscles because of too much stretching, unless you stretch so much that you tear muscle (which you will recognize via pain) and then do not recover (that is, nourish and sleep) enough to rebuild the muscle; this is unlikely. You are worrying too much.