Welcome to healthcaremagic community forum..
Where exactly is the lump located?? A lump located in the groin which disappears on lying down is possibly a hernia
. Hernia's are one of commonest condition presenting as lump in the abdomen. You will need an examination by a doctor, to deternine the content of the hernia and the mode of treatment.
My suggestion is you report to your nearest doctor for the physical examination.