Cyst help please

Patient: 20 years old. female. 5 foot 6. 8 stone. non smoker, no previous medical problems.I have a lump under my skin, it is in lower left abdomen near hip bone pelvic area. The lump is 1cm, hard, non tender, not mobile, and has been here for around 10 weeks. I have also not had period since 23 december where it was just spotting, and have always been regular never any problems. I am 100% not pregnant . I am healthy and do not take any medications or contraceptives. I feel I sometimes have quite bad anxiety but have not yet done anything about this. The lump itself is not painful, however the area around it and near my left rib sometimes is painful and uncomfortable. I have had blood and smear test done which came back normal. Seen my doctor yesterday and he said not to worry as it is probably cyst or lymph node which will go away itself. He did not say much else about it. So if it is this, why is it uncomfortable around left side and under my ribs? And why have I missed period when I am normally regular and healthy?