It feels like there's a lump inside my elbow. I can't straighten my elbow because of pain and the lump seems to move as the muscle moves. What is this?

Lumps of any kind must be examined and diagnosed by a medical doctor. This could be a benign tumor, ganglion cyst, lymph node, or more serious lesion. It isn't possible to tell without further testing.

Lymph nodes inside the elbow don't usually limit motion even when swollen. It could be a ganglion cyst since your motion is limited. A ganglion cyst is a pouch filled with fluid from the joint. A tiny hole in the joint capsule allows fluid to escape into a side pocket. A cyst forms to hold the fluid.

Benign fatty tumors called lipomas can also form a soft, moveable mass such as you describe. These aren't malignant but they can put pressure on the nerve or take up space in the joint causing problems.

Have it checked out before it gets any bigger. Early diagnosis and treatment for most conditions means a better result in the end.