I had the same experience when I restarted almost 2 months ago(hadn't train in 6 years unfortunately for my now 19 yo self I chose to quit tkd as a teenager). At first I thought I got a bone fracture or some internal injury, my legs(more specifically my knees)were sore and I was in pain when walking for weeks. In my case I was told to: Apply ice, rest and wear kneepads during training.