Yes, For Two Days Now, I Have Awakened With A

Yes, For two days now, I have awakened with a swollen left elbow. I have virtually no pain, I can push it lift with it but there is a red swelling around on the forearm the size of a half of golf ball. I called my Doc. but since it is Monday, they are swamped. I don't believe that I was bitten by anything as there is no stinger. A friend of mine who has "mersa" is worried that I have it. What could this be?????