Usual disclaimers about diagnosing over the internet apply.
If i understand you correctly - everyone has a lump there (called your tibial tuberosity), but sometimes it can become enlarged and/or painful.
One
possibility could be
Osgood-Schlatters. Most common in teenagers, but sometimes it doesn't resolve and still gives problems later on (possible, if as you suggest, this is an ongoing problem for you).
It's also possible for adults to get similar symptoms due to significant stress and/or trauma to the knee.