That could be her crop, can't make it out very well....That's where the food that goes in her mouth go to before the rest of the digestive tract. Surprised the feed store didn't have what you needed, thank goodness for Amazon
The 100 watt bulb is probably too warm, I used a regular 40watt bulb but have since converted to the MHP (heating pad). Careful about the bulbs sold, have read the teflon ones give off a toxic fume (invisible) that will kill the chicks.
I agree coccidiosis is a strong possibility. Aprolium (Corid) is critical if that is the case.
That bulge is her crop. Maybe she impacted her crop when she went outdoors. Is her crop emptying overnight? She may be starving, even though she is eating. Did she have access to grit? You may need to gently massage her crop, but take care not to aspirate her (don't let the contents come up and then go down her airway). Do you have any coconut oil on hand? If so, make sure it is solid and feed her little chunks. Don't force this. You can put little chunks in her beak.