Most of the time I would say it's abnormal. Anytime you feel shoulder pain it has the potential to be serious.
Make sure you are using correct form in your pressing. You also want to keep up the flexibility in your pec area, lats, front and rear delt regions.
A lack of proper flexibility in one region of the shoulder can shift some mechanical stress to another area.
If it keep up, see a doctor and get an x ray on the area. While I seriously doubt it's anythign to major, it is possible for many conditions to cause shoulder pain, not just the ac joint area.
Make sure you have adequate flexibility and that you're warming up thoroughly. If it persists, see a doctor.