Weight gain??

Water weight, food being retained in your bowels, PMS, etc...

There are a ton of different reasons for weight fluctuations and they happen to everyone.

Since you are 17 and in high school and don't have a lot of weight to lose and are very active you might want to reconsider your calorie intake. You should at least be eating back your exercise calories otherwise you may find yourself getting sluggish and fatigued. Just my opinion.

Why are you only eating 1200 cals and doing 5 days of cardio!?

STAHP!!!

Then go back to your doctor and get more "advice". Typically you need to eat above 1,200 to lose weight, especially if you're exercising.

You might want to talk to a Registered dietitian.

Unless you're like 80 lbs, or severely obese that is way too large of a deficit and usually always shunned upon. What are your stats?

You don't need to go all out to lose weight, plus you run the risk of severely damaging your metab. Imagine having to eat 1200 cals to maintain.

According to the calculator on here, Your estimated BMR is: 1,409 calories/day. That's how many calories you burn if you stay in bed all day and do nothing at all. I assume you don't do that, soo you should ideally be eating somewhere around that.

I'm also also nervous to increase my calorie intake at this point three weeks into the diet, as I am worried I have slowed down my metabolism so much that I might gain weight back the second I increase calories at all.

Ok but if you are slowing down your metabolism by not eating calories (which you are) and raising the risk for potential bodily injury (which you are) then eventually you will have to add calories. I would just increase caloric intake now. You could sit at 1400-1500 calories per day, drop weight and you won't bonk during a workout. Also don't do solid state cardio all week. Do resistance training that will put you in better shape MUCH faster. Don't be afraid to lift free weights you won't gain muscle like a man does..not enough testosterone.

PS. Stop watching the scale you will gain weight by stressing over a gained 2 pounds that is most likely attributed to water weight. Water weight can fluctuate by up to 5 lbs each day. Feel how your clothes fit.

I'm also also nervous to increase my calorie intake at this point three weeks into the diet, as I am worried I have slowed down my metabolism so much that I might gain weight back the second I increase calories at all.

Since you are only 3 weeks into the diet you won't run any risk by at least eating up to 1600 calories a day, which is what would be a recommended minimum. Aim to lose 1-2 pounds per week, but ideally 1 to 1 1/2. Anything more than that & you could cause a major slow down of your metabolism. Over time a very restricted diet causes a slow metabolism and in turn could cause more fat gain, lean muscle mass loss and make it even harder to lose that weight. Speak to a nutritional coach or registered dietitian for healthy eating guidelines. The obvious is no fast food, processed box foods and sweets like candy, cola's etc. But as for your Basal Metabolic Rate, aka BMR, you need to take in more than 1200.

http://www.webmd.com/diet/healthtool-metabolism-calculator

Using this calculator, and with the info you provided I came up with these numbers:

Inactive - You Burn 1698 Calories Each Day
Lightly Active - You Burn 1946 Calories Each Day
Moderately Active: You Burn 2194 Calories Each Day
Very Active - You Burn 2442 Calories Each Day
Extremely Active - You Burn 2689 Calories Each Day

Definitions of Activity Level
Inactive: little or no exercise
Lightly Active: light exercise or sports 1-3 days/week
Moderately Active: moderate exercise or sports 3-5 days/week
Very Active: hard exercise or sports 6-7 days/week
Extremely Active: very hard daily exercise or sports; or physical job or 2X day training

Increasing your activity level will boost your metabolic rate. To lose weight, decrease calories consumed and/or increase calories used by about 500 calories per day.

Based in what you have told us, you should be eating around 1694 calories per day.

You would do even better if you added a couple days of weight training to build up lean muscle mass which increases your BMR & burns more fat at rest, and it will keep you toned & strong. All cardio & no weights = Skinny fat, which trust me, isn't what you want.

I learned from my mistakes, endless hours of continued research and college studies. If you under eat you won't lose much fat. Also keep in mind that it is perfectly normal for water weight to fluctuate up to 10 pounds, depending on your diet, hydration, hormonal fluctuations and sodium intake.

Good job on the running & making the decision to become healthy. I hope you continue to do well with your goals :flowerforyou:

please eat more

This ^^ and what others are saying. When you say your doctor, is he/she a license nutritionist? 1,200 calories is just not enough... You are eating way too little. Don't increase calories by eating ****, eat more healthy things. Give it a shot, it definitely made a big change for me!