You can use the expressed breast milk which is kept at room temperature for 6 hours. In case you keep it in refrigerator at 25 degree C, you can use it for 24 hours. Breast milk which is frozen can be stored for 3 months. Don't freeze the breast milk, again and again. I don't think rashes are occurring due to your feeding practices. This is infantile allergy, which is peculiar to babies below one year of age, and tends to settle with time. If it is severe, you may intermittently use creams prescribed by your paediatrician. Minimise the use of soaps.