On birth control for less than seven days, Will it affect the body?

Patient: I’m a student athlete who runs track and I’ve been taking birth control pills for my extreme period pain.I started the pills the first day of my menstrual cycle.I’ve been on them less than a week, but my coach is worried they are starting to affect my body and running.How long does it take for side effects of birth control, such as weight gain and water retention, to have an affect on the body?

Doctor: