This is due to the way you carry your baby. It's called tendonitis. I had it also. Went for steroid jabs @ singapore general hospital to ease the pain. It was so painful I was crying everyday.
End up I got to a chinese physician at People's Park to massage. Much better than the steroid jabs. I also try not to carry my boy too often.
The other day my boy was throwing a tantrum when I was carrying him, now end up my right elbow also got this problem.