Is it usual to get a breast infection after radiation?

Possible. Radiation treatment can cause significant changes to the skin and breast tissue including ulcerations, redness, and long-term problems such as fibrosis (hardening). These changes can look similar to infection but usually are not; ask your radiation oncologist.
No. Getting an infection in the breast after external beam radiation is very rare. It happens in only 1% of patients or less. Infections can be more common after partial breast brachytherapy where a device is left in the breast penetrating through the skin. Infections probably occur in about 10% of women getting this treatment, but they usually are easily controlled with antibiotics.