Penetrating the skin always causes some kind of tissue reaction. Repeatedly using the same needle and same injection site aggravates the tissue reaction. Insulin in and of itself may also increase the fat tissue. It is typical that too small area is used for insulin injections. When injecting, you should leave at least a couple of finger-breadths between the previous site and the new one.
The injection sites and insulin pump cannula sites are regularly checked at the appointment. Any hardening or swelling which is not visible is located by feeling the injection sites. You should also keep an eye on the condition of the injection sites yourself and make sure that there are no emerging calluses or swelling. The most important thing is to use a wide range of injection sites and change the needle every time you inject and cannula regularly.