Common problem. It happens when air under the shower is being warmed and tries to rise. This pulls the shower curtains in toward the person. One remedy...If there's a tiled wall where it doesn't matter if water gets on it, draw the curtains aside next to the wall to leave a gap so that air can enter. This will prevent the curtains being drawn in.
Alternatively, instead of heavier fabric, some shower curtains have lead weights (like big lead washers) sewn into the bottom edge.