Did I mess myself up and is the pounds I lost muscle?

So you've lost 15 pounds in roughly 4 weeks, about 3.75 pounds a week? Yes, that's fast loss, even at your current weight.

Set your MFP activity level based on your activity excluding intentional exercise, just reflecting your job, home chores, non-exercise hobbies and that sort of thing.

Tell MFP you want a weight loss rate in the range of 0.5% to 1% of current weight per week, with a bias toward the lower end of that range unless so severely obese that weight is a health threat, in which case it would be good to be under close medical supervision for deficiencies or complications. For you now, that would be around 1.1-2.3 pounds per week, so if it were me I'd set it for 1 or 1.5 pound weekly loss for now.

When you exercise, log the exercise and eat at least a fair fraction of those calories, too. Some people worry that exercise calories are over-estimated so eat back 50-75%. I'd strongly suggest picking a standard percentage to eat back every time during your initial trial of this calorie level, whether it's 100% or something less. (A standard percent makes it easier to adjust intelligently later if necessary.)

Follow that calorie goal reasonably closely on average for 4-6 weeks. If female of the relevant age/stage, follow it at least one full menstrual cycle, to compare body weight at the same relative point in at least two different cycles. By "reasonably closely on average", I mean roughly +/- 50ish calories, when you look at your weekly averages in the MFP app. (You don't have to be that close every day. Bodies don't reset at midnight, even though MFP does.)

After that trial period, calculate your average weight loss per week. If that's dramatically different from your weight loss target rate, adjust using the assumption that 500 calories a day is a pound a week (use arithmetic for fractional pounds).

Have you lost more than minimum muscle? There's no way for anyone else to say. But in a month, honestly probably not very much. If you were getting ample protein and doing challenging strength exercise, even more likely that muscle loss was minimal.

The background info:

Most of us burn the biggest fraction of our daily calories just by being alive. That's BMR, basal metabolic rate. Sometimes it's estimated as RMR, resting metabolic rate, which is a very close number with a slightly different definition. The calorie burn comes from breathing, heart beat, blood circulation, gazillions of cells doing biochemical/physiological stuff, brain doing its thing, etc.

For most people, the next biggest fraction of daily calorie burn is non-exercise activity thermogenesis (NEAT), the calories we burn doing daily life stuff like our job, home chores, etc.

In common cases - which may not be exactly true for you - exercise comes in 3rd in relative magnitude of calorie burn. That's sometimes called exercise activity thermogenesis, or EAT.

There are some other small things. The one people sometimes mention is the thermic efficiency of food, TEF. That's the calories burned as we digest/metabolize food. It's not very many calories in practice, but sometimes people talk about it because there are small difference between food types.

Add all those things up, and you have total daily energy expenditure, TDEE, the calories you burn all day in every way. Approximately:

TDEE = BMR + NEAT + EAT + TEF

Eat less than TDEE, we'll lose weight. In reality, TDEE varies daily, but much of calorie counting assumes estimated averaged values of BMR, NEAT and TEF. Some people average exercise plans into their calorie estimate (use a TDEE estimate, don't log exercise separately). MFP estimates BMR + NEAT + TEF, subtracts some calories if we asked for a non-zero weight loss rate, and gives us a base calorie goal, to which we add exercise calories when we exercise. That process keeps the same calorie deficit for the same estimated weight loss rate.

If you have lost more than minimum muscle in the past month, guess what: It's water under the bridge. Don't worry about it. There's no point.

Going forward, stick with a more moderate weight loss target, get ample protein (plus other good nutrition overall, ideally), fuel your exercise (for best performance and effectiveness), do some progressively strength-challenging exercise. You'll do fine, long term, I predict.

Above is just my advice based on successful loss/maintenance with MFP. Others will comment and agree or disagree, so you can decide your best course.

Best wishes!