Please do not worry - your data is safe! Your workouts are stored in online database as well so your temporary user session issues won't cause loss of data.
Your workouts should appear back in the app if try to log out from the Swimmo application and log in again. Go to Swimmo mobile app → Settings → About me → and then tap "LOG OUT" button.
If this didn't help please log out again from the Swimmo app, go to phone application settings in your phone Settings section and use "Clear app settings" option. Check here how to do it on Android. Then log in again. Your workouts will be visible again.