Hello Mukku92,
Thank you for reaching out in Apple Support Communities. Our recommendation is to restart your iPhone. This is a step that can help resolve unexpected behavior. Once your iPhone has been restarted, check and see if Fitness app encounters this same behavior. If needed, here's how to restart your iPhone:
How to restart your iPhone X, 11, 12, 13, or later
1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
This information is outlined here if you need to reference it: Restart your iPhone - Apple Support
Kind regards.