Any baby diaper rash ointment should work. Vaseline also works pretty well.
Ensure that her bottom is well cleaned after she has emptied her bowels (use a wet wipe and dry the bottom, or use a dampened toilet tissue paper). Use the diaper rash ointment, Sudocrem or an ointment with zinc, your pharmacist will also be able to assist to help you choose one.
If the problem does not start to improve within a couple of days, get a doctor to check her over. Sometimes soreness can be caused by a fungal infection.