Whats wrong with me?? Help?

I have recently gone on a diet exercise plan my doctors say i am overweight for a 13/14 year old girl (i prefer not to tell my weight publicly) and so i am on a strict diet of 1886 calories a day with some sort of physical activity ranging from 20 minutes up to 3 hours. I have been doing this for 4 days. For diet i have been staying within calories or lesd but have been exercising day 1 i swam for 5 hours and day 2 two and a half hours day three dance for a hour today basketball 30 mins and 1.898 mile run. I have noticed today and yesterday i have been having alot of cramping and stomachaches could this be due to what i am doing to lose weight? And i am not due for my period for another 2 weeks