Hello, when did this come on?
What is your general health?
Do you take any medication?
Are you a smoker? Any long distance travel recently?
How swollen is the leg?
A blood clot usually comes on relatively quickly, and would have caused severe pain and swelling after a period of time however it cannot be ruled out completely without a scan of your legs.
Localized pain to one area may be an insect bite, or a bacterial infection including cellulitis so if the area is warm and red then it needs to be looked at by urgent care or ER.
It could also be localized inflammation of the blood vessels, and in that case you might see prominent vessels, or may be tender to touch. This also needs treatment at urgent care.
The other possibility is that this is a ruptured bakers cyst (these appear behind the knee and can rupture leading to swelling in the calf. They are painful but harmless).
I am sorry that I cannot be more specific, the history does not quite fit with a blood clot and ideally this needs to be examined in person before deciding on appropriate treatment.