If the doctor said DON'T worry then don't worry...
Echogenic material in a fetus's stomach can be seen in ultrasound and most times its usually nothing. It could be swallowed vernix caseosa, or epithelial clumps. Don't panic. If the doctor says don't worry most times its nothing.
Did they schedule a follow up ultrasound? If they suspected something that would require you to have some amount of worry, I do believe they would have said what it was. In this case, I wouldn't worry. I speak from experience.
Yes I just had an ultrasound follow up yesterday... Doctors didnt see spot anymore so they say everything looks great and nothing for me to worry about