My 3 (almost 4) yr old son had the stomach flu last weekend. He seemed fine for a day or two but?

he has still thrown up out of nowhere a few times. Actually my husband and I both got the same stomach bug, it lasted about 2 1/2 days but now we are fine. My son had been healthy for about 24 hrs and then threw up. I chalked it up to residual illness. Then this morning, after being fine since Wednesday, he threw up his breakfast. He was fine all day, running around, eating normal, BMs back to normal until about an hour ago. He woke up crying because he had thrown up in his bed. I am wondering if this is still just residual illness, if it's the fact that he was on a liquid diet for 2 days and his stomach still isn't used to a normal diet or if there is something else wrong. He is getting plenty of fluids, eating healthy and otherwise is totally fine. I plan on calling the Dr on Monday if the vomiting continues but I am wondering if I should call the emergency call line tomorrow or just wait it out. Thanks!