you need to isolate the rotations by using pushMatrix() and popMatrix();

Another example (probably not what you want, but you see the principle)

Greetings  

1. float a;
2. float fx, fy;
3. float b;
4. void setup() {
5.   size(640, 550);
6.   smooth();
7.   fill(255);
8.   rectMode(CENTER);
9. }
10. void draw() {
11.   background(102);
12.   // -------------
13.   pushMatrix();
14.   translate(320, 275);
15.   a = atan2(mouseY-height/2, mouseX-width/2);
16.   rotate(a);
17.   rectDraw(250, 50, 0.45, 0);
18.   popMatrix();
19.   // --------------
20.   pushMatrix();
21.   translate(320, 275);
22.   rotate(b); 
23.   rect(0, 0, 50, 50);
24.   b+=.1;
25.   popMatrix();
26. }
27. void rectDraw(int r, int s, float c, float d)
28. {
29.   for (float i=0; i<2*PI; i=i+c)
30.   {
31.     fx = r*cos(i);
32.     fy = r*sin(i);
33.     rect(fx, fy, s, s);
34.   }
35. }