Is there a significant mismatch between your upper and lower jaw? As long as the bite remains functional, the mandible should automatically adapt to the expanding upper arch by widening sufficiently. In this way, lower jaw expansion tends to be redundant.

Well, the thing is that I had two teeth removed from my lower jaw, and I feel that my face shape is too triangular with my lower jaw being narrow. 

Do you think using a lower jaw expander may help the lower jaw?

I would be afraid of trying to expand the lower jaw with a device! Everything seems to be hinged on the upper jaw/maxilla, and everything changes when it changes. But I don't think expanding the lower jaw has the same effect, you're just expanding the lower jaw and it doesn't cause the upper to expand.

The bottom jaw is hinged outside of the upper jaw. If you just expand the bottom it could be trouble. Don't get TMJ!