MPU9150 on arduino (ROS node) #3

For starters thanks for your library!

I have been trying to use your library to get data from an IMU connected to an arduino UNO. Moreover, the sketch on the arduino will work as an ROS node (don't know if you are familiar with ROS).
After some problems I was able to get the IMU to work and I was able to receive the IMU data published by the arduino. However I want to use the same arduino to control some motors using an H-bridge. When I implement the subscriber (basically a function that will get the controls from the computer and execute the necessary motor control) the IMU stopped working.

At that point the output of mympu_open() is zero, while the output of mympu_update() becomes "-1" and I'm unable to read the IMU.
Can you help me understand what might be going on?

Note: I'm using the MPU9150