Does your leg feel warm to the touch? Or is the warmth something you feel within the leg, without touching it?
I ask because warmth to the touch is one of the symptoms of DVT (deep vein thrombosis.)
You might be able to find that symptom in an online "symptom checker." (These can be found by googling that phrase. There are a number of them online.)
If you do find you have symptoms of DVT, get it checked out immediately, either calling your doctor immediately or going to the ER for an ultrasound, as DVT can cause a pulmonary embolism, which is serious and needs to be prevented.)
It's great you are healthy and active. DVT arises typically with not moving about for long periods, so fortunately that doesn't appear to be your situation, but I hope you'll check a symptom checker.
Perhaps you're just a "warm leg, warm heart" person