Arnica Montana is probably the first remedy anyone interested in homeopathy should know about. Arnica is useful for the symptoms of bruising and other blunt force traumas, muscle soreness after working out, and shock after any kind of accident (car accident, bike accident, etc). It's one of the few homeopathic remedies that have become "mainstream"; arnica gel is now commonly used by olymplic athletes.
Arnica will often be beneficial if the person has some of the following symptoms:
-Aching pains, as if beaten.
-Pains worse from any motion
-Any symptom worse from becoming cold
-Nosebleeds and pain as if the nose were crushed.
-Hot face and cold body
(All information summarized from Boenninghausen's Characteristics)
So if you have been hitting the gym harder than normal, try taking some Arnica and see if it helps.