Your calorie goal may change because you have Energy Expenditure is set to Activity Tracking. With this setting enabled, your total calorie goal will change accordingly based on your activity. For example, if you burned 400 calories walking with a 1200-calorie goal set, your calorie goal will increase to 1600 after exercising.
If you don't like that approach, you can change the settings at any time. Just go to your Profile in the upper-left corner, tap "Energy Expenditure", and select "Estimation" mode. This will set your calorie goal based on the activity level you provided in the questionnaire.
