A lower ride does not necessarily indicate that you have an aftermarket suspension (for better or worse).
1. You could have cracked springs (yes....somehow in BMW's infinite wisdom, they engineered a spring that is prone to cracking).
2. Or your shocks could be blown...effectively lowering your ride because they are not rebounding properly.
3. Or, you could have aftermarket springs....
Have you actually gone under the car (or taken a wheel off) and looked at the springs?