Large, repeated spots on many frames of a 35mm roll are usually not caused by sensor dust, since film cameras don’t have sensors. Possible causes include debris somewhere in the camera or lens, a camera fault such as a shutter problem, or a lab/processing issue. If the spots appear in the same place on frame after frame, that can point to something in the optical path or camera.

In your case, the most likely explanation is what you later identified: opening the camera while the film was still inside. Exposing the film to light can fog or mark much more than just one or two frames, especially depending on how much film had already been wound and how long the back was open.

Before shooting another roll, check the film chamber and seals for anything unusual. If the problem happens again on a fresh roll without opening the camera, then processing or a camera/shutter issue would be the next things to investigate.