That is an excellent question and one that is best answered by your primary care doctor
or another specialist. In general, there are several causes for warm leg sensations and because of the limited history provided, it is impossible to know what the specific cause is for you, but your doctor will be able to know the cause with a complete physical examination and history. Some potential causes for warm leg are venous stasis in your leg which can be possibly caused by a deep venous thrombosis which is causing blood to return to the heart slower than what would be normal. This could be a serious issue if that is the cause of your symptoms, especially if the DVT is above the knee as there is an increased risk of pulmonary embolism related to this. For this reason, it is very important that you see a doctor in an urgent basis to find out whether this is the case or another underlying cause. Another possibility is that there is an underlying infection that is brewing in that leg as this can sometimes show itself as warmth in the leg, but again only a doctor will know the cause for your symptoms.