What Causes Severe Pain And Swelling In The Left Elbow?

I had a severe pain in my left elbow and found it to be swollen and had 2 der lines going to my hand. Pain was severe and the extreme pain when touched. I also had lines that are still visible slightly. The Dr. saw me right away and they did an ultra-sound. Also gave me an injection of antibiotic and anti-inflammatory before I left the office. They never did explain what it was. All I got was the good news there was no blood clot. I m looking at the word Thrombocytopenia and was never told what it was or how I woke up with my arm unusable and in that condition. Can you tell me anything else ??