After having my transmission fluid flush, the next day while driving I hear a thumb and my car

After having my transmission fluid flush, the next day while driving I can to a stop light, then began to accelerate into first gear, while doing that I hear a thumb and my car. After passing the first gear my car runs great. If I get on the highway and drive for a while then come to a stop light, began to accelerate it doesn't happen while moving into first gear. Only after making few stops then it happens again