Are you taking statins also? Statins can cause muscle pain and loss. If you are, there is a recommendation to take CoQ10 supplements as well.
I take both statins and metoprolol tartrate, also at 12.5 mg twice per day. I doubt my pains are due to the latter, but I've never tried eliminating them. I also cut mine to half dose because of the effect it had on my heartbeat.