Lump in Abdomen

I am a 23 year old female, with a lump on my upper left side of the abdomen, right under my ribcage. The lump is only there sometimes, it will tighten into a hard long lump, then I stretch out and rub it lightly and it goes away. It comes if I turn awkwardly, sneeze, cough too hard, or exercise. I have had this happen randomly since I was 11 years old. The spot where the lump formed is always tender afterwards, for a few hours... but the lump is no longer there, just the soreness. I believed this to be a hernia... but I was wondering if anyone else had any ideas as to what it could be. Possibly a muscle spasm?