It's probably just from impact from skating. Don't skate for a bit and keep it iced for about 20 minutes at a time and keep it elevated. Next time you skate don't go too hard and it should get better. If it still hurts the same in about 2 weeks I would consider seeing a doctor.
i have a really bad left knee, i blew it out on a rail about 6 months ago. what i would recommend doing is ice and heat. its worked the best for me. I iced and elevated for 20 minutes then to a heat pad for another 20 then let it sit... then rinse and repeat. After you skate you want to always ice first and then heat.