Does the pain feel like an injury-type pain? Or is it Delayed Onset Muscle Soreness (DOMS) similar to the feeling the day after a hard strength workout?
If the former, I’d get it checked out by a doctor or physio.
If the latter, it sounds normal. I’ve heard that impact forces when running downhill can exceed 7x bodyweight, plus the load is absorbed eccentrically (while the muscle lengthens), so it can be a huge load on the quads.
I usually have sore quads if I haven’t been doing any downhill running and then start up again. It usually takes me two or three spaced outings to stop the sore quads feeling. Easing into it seems to work the best: an intentional super-easy decent to start with, building it up slowly over subsequent outings, and then consistent downhill running (at least once a week).
Hope that helps.