Lumps did not show up on ultrasound!

For the past year, I've been experiencing fullness in my stomach and nausea with a slight amount of pain. This then led to the development of a few lumps in my abdomen- but I didn't think much of it because they didn't really hurt or anything...but now, I have several bigger and harder lumps which now have spread further up my stomach and below the pelvic area. They also hurt quite significantly and sometimes the pain is incredibly sharp and I just can't cope with it. I went to have an ultrasound and I was certain that they'd find my lumps...but the scan didn't find anything and they are most definitely there and painful. I don't know what to do :( I'm eighteen so I doubt it would be anything very serious but I'm worried and the symptoms get worse. Along side these bumps, I've been experiencing fatigue and weakness. I feel stuck and don't know what to do! I'll be going back to the doctors anyway but they often send me away and say it will probably go on its own (don't have a very good doctor). Please help me :( !!!!!! It affects my day to day life and it needs to stop.