Last time I played tennis was back in October 2017. But I have an illeostomy that will hopefully be reversed this summer. Being very active with a part of your body sticking out of you seems a bit dangerous for me. I'm a runner too and the bag presents some problems as does chemo. So I'm a ways from being able to get back to normal training.
I would ask your surgeon for advice. Of course she gave me advice and I did ignore some of it (like not lifting more than 10 pounds the first month). But it would be at least nice to get their guideline.
If you had surgery at a major cancer center, you might ask if there's a doctor that specializes in sports for those recovering from your type of surgery. My fitness center manager had surgery for a hernia recently and gave me the name of a sports PT doctor that I plan to talk to after surgery. This doctor specializes in getting people back into shape after injuries. I was actually surprised that my fitness center manager consulted with such a person as she's already highly qualified herself.
At 21, it's pretty hard not to be active and you may feel invincible and your body likely heals far faster than most here.
6/23/17: ER rectal bleeding; Colonoscopy
7/13: Stage 3B rectal. T3N1bM0. 5.2 x 4.5 x 4.3 cm. Lymphs: 6 x 4 mm, 8 x 6, 5 x 5
7/31-9/8: Xeloda 3,400 mg/day+radiation
7/5: CEA 2.7; 8/16: 1.9; 9/8: 1.8; 11/30: 0.6; 12/20 1.4; 1/10 1.8; 1/31 2.2; 2/28 2.6; 4/10 2.8
MSS, KRAS G12D
10/6: 2.7 x 2.2 x 1.6 cm (-90%). Lymphs: 3 x 3 mm (-62.5%), 4 x 3 (-75%), 5 x 3 (-40%). 5.1 CM from AV
10/30: LAR, Temp Ileostomy, Path Complete Response
12/20: Started CapeOx