How to lose fat and not muscle ?

2 weeks isn't long enough to notice much of anything that you could discern anything from.
Except - if calories are too low for level of activity, your body can respond fast and slow you down to conserve calories.
So you can get tired that quick.

That doesn't mean you are losing muscle yet, just as getting stronger doesn't mean you are gaining muscle either.
But, if that is indeed the reason for being tired - yes you will lose muscle mass. Probably the normal 15-25% of weight being that.

How do you lose just fat and not muscle?
1 - Eat at reasonable deficit. More you have to lose, faster you can attempt with bigger deficit. With 20lbs to go, you should be at max 1 lb weekly if you exercise 3-4 days at least, if less than that, then 1/2 lb weekly.

2 - Get in enough protein. 0.82 grams per pound of body weight. Fat is 0.35 g / lb bw. Carbs gets whatever is left of your calories.

3 - Resistance training of some sort. What's used is maintained. In a diet, what's used most is maintained.

Those 3 things each by themselves has shown to retain muscle mass in research setting with lab measurements and constant Dr supervision. You don't have that - so do all 3 things.

To the end of keeping that reasonable deficit, MFP estimates what you'd burn with NO exercise - and then gives you an eating goal.
That is ONLY when you do NO exercise.

If you burned more in the day because you exercised, then you take off the same amount of deficit - you eat more.
So MFP just adds your exercise calories to your eating goal - you eat more.
But you are still eating less than you burn - you lose weight.

So use the tool right. Tools used wrong at best don't help you, at worst they can hurt you.
Log your strength training as that if it is indeed weight lifting with sets and rests. If it's fast machines or circuit style training, log it right that way.

2-3 lbs is normal water weight lost when you start watching sodium and eat in a deficit.
So you might also confirm you are logging food correctly - by weight, not measurements, which is for liquids only. Calories is per weight - grams.

Not eating enough would cause loss of energy. Can you open your diary?

No one else has posted link, so here you go...how to get you started on your path to sexypants. I go back to this on a regular basis.

http://www.myfitnesspal.com/topics/show/1080242-a-guide-to-get-you-started-on-your-path-to-sexypants

If you truly are losing strength (though, I would argue that 2 weeks is too short a time period to come to any conclusions), you need to eat a bit more.

If you're not allergic to lactose (milk) you can give proteine shakes/bars a try to preserve muscle mass

You'll have lost mostly water so far but don't fear the carbs. MFP gives you a guide to what macros to hit. Exercise will help you retain muscle. How much are you looking to lose?

The two best and most straight forward pieces of advice for this that I can tell you are
1) Eat protien
2) Lift weights!