My 9months baby has 3 different lumps at the back of her head and it seems to increase in size what can this mean?

If they are appearing along the hair-line at the back of the head, they are most likely lymph nodes. If your bably has had colds, of especially ear infections, the lymph nodes are growing to better filter any "poisons" from the infections and keep it from spreading... It would be a good idea to have your doctor, PA or NP check the baby to make sure that is what is happening.