Hi sir/madam, It can be really a cause for concern to have a lump behind ear. But don t worry its definitely not a serious problem and also not mastoid inflammation. Most of the times, it would be a lymph node. Lymph nodes are involved in our body s defense and are present all over body including behind ears.They increase in size whenever nearby areas are exposed to infections. For example, cradle cap of head can cause lymph node enlargement. Usually lymph node swellings are painless and occur on one side. However there are other rare conditions leading to swelling behind ears. Its always better to get an examination done by a doctor. Take care.