Why do I have no swelling ociated with my achilles tendinitis?

Yesterday, I woke up with my foot slightly hurting. As the day progressed, the pain became more and more intense. I ended up going to the hospital and the doctor diagnosed me with achilles tendinitis. I have no swelling and can't recall doing anything to hurt my foot. All I know is it hurts...BAD. The doctor wants me to follow up with a specialist. I just don't understand how I can have an injury that may require surgery with no visible swelling or bruising. Has anyone ever dealt with this before or know anything about it?