The combination of the lump you describe and the abdominal pain make me quite concerned about a potentially serious infection, particularly PID or conceivably a vaginal wall abscess. It may or may not be related to the missing tampon, and it may or may not be gonorrhea, chlamydia, or some other STD. Please see a health care provider right away. Don't let embarassment about the tampon delay getting care; delayed treatment of pelvic infection can be very dangerous.
Post a follow-up and let me know what happens. Good luck-- HHH, MD
A tampon can't get permanently lost inside you, but it could get pushed up toward the cervix and be difficult to reach. If you think it is still there, and can't get it yourself, then you need to see a doctor to have it removed. Don't be embarrassed; this has happened to many women and gynecologists are used to dealing with it. But please don't just leave it because you could develop an infection.