gringol, It MIGHT be due to the string pinching the finger down onto the nock. There also MIGHT be several reasons for this so here are a couple ideas.
First, when you place your fingers on the string,keep the index finger above the nock a little (maybe about 1/8 inch). Don't start with it right on the nock.
Second, (more of a question) have you tried a finger spacer on your tab (if you shoot a tab)? That will help hold the index finger up off the nock.
Finally, be sure that you have a relaxed string hand and wrist and allow the weight of the bow to align your hand, don't try to "muscle" your hand to align the string.
These are just ideas to try. Bottom line a callus is telling you something is not correct.
Arne