This is quite common in active kids!
It is called Sever's disease
Sever’s disease is a painful condition of the heel that occurs in growing children. It happens when the tendon that attaches to the back of the heel (the Achilles tendon) pulls on the growth plate (the apophysis) of the bone of the heel (the calcaneus). The repeated stress on the growth plate causes pain and inflammation at that site.
It most commonly occurs in physically active children between the ages of 8 and 14 years of age. This pain is often worsened by physical activity or when Achilles tendons are tight.
Rest: Limit physical activities, especially running and jumping, while having pain. Take rest breaks between activities.
Ice: Ice the heel for 15 to 20 minutes when having pain or after activities. Do not place ice directly on skin. Put a towel or other cloth between the ice and the skin.
Pain Relief: Anti-inflammatory medicines, such as ibuprofen (Motrin® or Advil®) or naproxen (Aleve®) can be used as needed for pain and inflammation (swelling). Always give your child these medicines with food. Be sure to read the directions on the bottle to give the correct dosage for your child’s height and weight.
Footwear: Always wear a well-cushioned pair of supportive shoes. Do not walk in bare feet. In some cases, special gel inserts for the shoe called heel cups may help.
Stretches: Stretching the calf muscles can help loosen tight Achilles tendons and help decrease the pull of the tendons on the growth plates. Some easy stretching exercises are shown below.
Do these stretches along with your child. This can make it more fun for both of you and may help your child want to do them.
Recovery from Sever’s disease varies from patient to patient. In most cases, Sever’s disease goes away on its own with rest, treatment, and time. Symptoms may worsen if your child tries to play through the pain or if proper treatment is not followed.
Your child may increase activity when symptoms have subsided. Sever’s is a disease, however, that can come and go.
Your child should continue stretches regularly to try to help prevent re-occurrences. Sever’s disease typically goes away completely when the growth plates close.
You can read more here
FYI : I had the EXACT same problem when I was 10 years age, and an active tennis player to boot :)
I just rested for 15 days, read a lot of books, and was fine ..