p5.js - get a rectangle to move left and right repeatedly (bounce)

I'm trying out some sample code for a bigger project, and I'm having trouble getting my rectangle to bounce between two lines.

function draw() {
    print(frameCount)
    background(255)
    var x = 150 + frameCount;
    rect(x,200,15,15);
    line(150,0,150,400);
    line(250,0,250,400);
    if (x >= 250) {
        background(255)
        x = 350-frameCount;
        rect(x,200,15,15);
        line(250,0,250,400);
        line(150,0,150,400);
    } if (x <= 145) {
        background(255)
        x = 145 + (frameCount % 100);
        rect(x,200,15,15);
        line(250,0,250,400);
        line(150,0,150,400);
    }
}

I'm getting the feeling that after the first instance, it's disregarding the original if statement, which dictates a bounce to the left. I'm really not sure what's going wrong, and any help would be appreciated.