A pic. would be good. I think you are describing an unabsorbed yolk sac remnant.
If there is a yolk sac remnant what I usually do after enlisting help to hold he chick is a) make sure the bulge is clean,cooled boiled water and soft gauze if it is dirty b) lubricate little finger with sterile lubricant c) gently reduce yolk sac through the hole (if hole is bigger than your little finger tip the chick is unlikely to survive) d) stick a band aid or similar across tightly to keep the sac in and do not touch again. Only apply a fresh dressing again if there is a scab etc. which others may peck.
Good Luck,
sandie