If you don't have an appointment with an orthopedic doctor, preferably one who specializes in hand/wrist/elbow/shoulder, then you need should get on the phone and make an appointment ASAP. Strange if you haven't done this already.
Interesting that as a right-handed player, your LEFT arm is bothering you. Normally, the serving arm is the one that gets injured, at least in my experience. OTOH, I am NOT a doctor, so take this advice with a grain of salt :-)
Possible causes I could think of:
Too large a grip
Incorrect technique
Too many off-center hits
Failure to warm up slowly
Overuse
Failure to strengthen the muscle area involved
Holding the racket too tightly
Traumatic blow to the injured area
http://tennismd.com/
is a nice basic web site for tennis injuries.
A couple of books on tennis injuries: