Thanks for your query.Based on the facts that you have posted it appears that you have either Spagelian Hernia
or Patent Uracus.
You have already done CT scan
scanning of the abdomen and ruled out Spagelian hernia.
Since the lump occurs in the morning when your bladder is full and goes away after passing urine I think possibility of patent Uracus which is a congenital remnant of uracus .
Please consult qualified Urologist
for endoscopic examination of bladder nad radiological test like radio contrast Cystogram of a full bladder.