I have been experiencing a warm sensation in my left calf for about a week. what could this be a symptom of?

Several possibilitie: Muscle strain, knee problems, blood clot are three possibilities .You should be evaluated sooner than later.

Non-specific: Warm sensation without other symptoms is a non-specific symptom. When associated with swelling, redness then this may suggest deep venous thrombosis, infection. Should the symptom persist for several weeks despite doing simple things like stretching, taking anti-inflammatory agents, massage, see your doctor.

Knee calf muscle: Besides what was already stated suggest evaluation of the knee or calf muscle.