I have a lump on lower back of my head after Hair Transplant. Any suggestion?

You need a face to face exam with a doctor.  Follow up with your doctor and explain your concerns.The Internet is not a place to receive a medical diagnosis.It can be something as simple as a hematoma, but it can also be something compeletely unreated. 

As the other doctors mentioned in their responses, you should have this checked as soon as possible. In my experience, likely it is a reactive lymph node and it should not be anything serious and should resolve by itself. However, it is best to check it.

is very sensative after a transplant and the device used can also make a difference i.e neograft or artas.  It is best to follow up with your doctor so they can take a close look at this situation.

In these kind of cases where there is a concern, you have to see your hair transplant doctor to make sure it is not something serious.  Some times it may just Be an ingrown hair that needs to be taken care of. 

Your lump sounds like a hematoma, a bleed below the surface of the scalp. This usually hardens and could be caused by the needle anesthetic injections you got. If there is pain there and it does not go away with heat, then see your doctor.

This could be anything from infection, hematoma or  an unrelated tumor. See your surgeon if possible or your general doctor. 

It is impossible to give you a diagnosis without examining you so you should immediately go to your physician (in person - this is not the time to Skype)!  That being said, the most common causes for lumps on the head are usually benign and self-limited.  If it is not part of your scar, you may have an ingrown hair or a sebaceaous cyst.  Either of these are very common and may be unrelated to the surgery.Go see your doc and good luck!