Breast milk should NOT be heated in the microwave because it loses its nutrients. When you heat it in a microwave you are "cooking" it, hence the cottage cheese consistency
You may be overheating it and it is curdling. If you are freezing the milk, set it in a bowl of warm water and allow it to come up to at least room temp before feeding. As the baby drinks, the milk will warm up further.
You should not be heating your breast milk in the microwave. You should gently warm it up under a stream of warm water from the tap, or by putting the bottle into boiling water. The microwave will destroy lots of the anitbodies and goodness in the milk.
