Hi eweeliina,
Thanks for posting in Apple Support Communities.
We understand that your Apple Watch is reading your heart rate while it's in your pocket. If you haven't done so already, restart your iPhone and make sure that your Apple Watch and paired iPhone are running the latest software updates.
1. Make sure that your watch is connected to Wi-Fi.
2. On your watch, open the Settings app.
3. Tap General > Software Update.
4. Tap Install if a software update is available, then follow the onscreen instructions.
Update your Apple Watch
To improve accuracy, calibrate Apple Watch.
1. While wearing your Apple Watch, go to a flat, open outdoor area that offers good GPS reception and clear skies.
2. If you have Apple Watch Series 2 or later, you just need your Apple Watch. If you have Apple Watch Series 1 or earlier, bring your iPhone for GPS. Hold your iPhone in your hand, or wear it on an armband or waistband.
3. Open the Workout app. To start, tap Outdoor Walk or Outdoor Run. To set a goal before starting, tap 
.
4. Walk or run at your normal pace for about 20 minutes.
Find these and additional steps in Calibrating your Apple Watch for improved Workout and Activity accuracy.
Regards.
