thank you for your query.
A lump behind the ear can be caused by a lot of conditions-
These lumps can be hard, painful or in other cases painless and soft to touch.
Lymph nodes are very reactive to touching. So the more you touch it the more it will stay enlarged. You don't need to put anything on it. It will get smaller on its own in time. Remember, the more it is touched the longer it will take to get to its normal size.
Also only if you have any additional symptoms like fever
,pain,swelling etc you can visit your GP for anti-biotics. Till then you can relax and wait for it to subside.
wishing you good health