I noticed a lump come up right below my left knee. it's kinda soft and moves around. it doesn't hurt. any ideas what it is?

Lipoma: There are many potential causes and I would not hesitate to be seen; but one of the more common causes of a non traumatic, painless and mobile mass is a lipoma. A lipoma is benign and is a growth of fatty tissues. An excision all biopsy can be done if there is any concern that something more is going on.

Cyst.: It could be a cyst , associated with a meniscal tear, or perhaps a lipoma, which is a non-harmful fatty deposit.