I don't eat enough

Maybe try some higher calorie foods like nuts, protein smoothies, some greek yogurt to help you get the calories in. I would definitely be concerned about not netting 1200 calories consistently after exercise. You may start experiencing fatigue and other health problems by eating so little.

are you trying to gain or lose?

If you honestly believe that you're not capable of eating a measly 1200 calories per day, then your exercise should not be cardio like Zumba.

But frankly, I don't believe that's the case. If your calorie goal is 1200 and you're doing Zumba, that tells me you want to lose weight. If you want to lose weight, then you're perfectly capable of eating >1200 per day. What were you eating before? Eat more of that.

If you are losing more calories than you can eat, it would be wise to either do less of that exercise or do a different type of exercise.
Before last week, I was only eating one meal a day (not quite sure how I'm still here, since I've never been above ~8 stone). Half of my problem before was that I was getting up in the afternoon (I'm a student lol) and didn't have enough time in the day to actually get hungry. You could try getting up earlier, like I have.

You're making some choices that sound an AWFUL LOT like someone who is trying to burn a lot of calories and eat very few calories, who is at least dimly aware that it's unhealthful, and seeking validation -- not a solution -- for it.

Like, if you think you're burning 600 calories doing zumba* and you say you're not actually that overweight, then you must be doing zumba for > 1 hour every time. A typical class would be 45 minutes, no? There's a broad spectrum between an sedentary lifestyle that you've characterized as "unhealthy" and doing 60 minutes of fairly heavy cardio every day. 30 minutes daily of moderate activity (for example, walking) is all that is really "prescribed" for basic, moderate fitness. Look, there's nothing wrong with working out hard -- it's great, I do it -- but if you're gonna go hard, you need to recognize that it's not just basic everyday fitness and you also fuel for it. Period. Refusing to eat a healthful diet and insisting on working out like that sounds like an ED in denial to me.

What foods, and how much, were you eating before you started doing this? Are you absolutely denying that you're eating differently now than then? A person your age even with a sedentary desk job would have been gradually losing weight at 900 calories per day if that has always been what you were doing, unless you are missing some body parts.

No sensible person is going to validate you for eating too little, or tell you that it's okay FOR YOU because you're a special snowflake. Not having a gallbladder is very common so NO, that's not a reason to eat very little food. I don't have a gallbladder either, for instance.

Also -- and this may be more to the point than anything -- more than half the time when people on MFP post this issue "OH I CAN'T EAT 1200 CALORIES IT'S TOO MUCH FOOOOOOOOOOD" what we end up seeing is that the poster thinks their calorie intake is low, but isn't measuring their portion sizes. Instead they're eyeballing portions and/or making weird assumptions. How are you measuring your portions of things for logging purposes? You really need to measure portions accurately (ideally by weighing everything that's not a liquid; liquids can be weighed or measured in a measuring cup) at least for 2-4 weeks to get a sense of what your intake truly is. It's crazy how much those "little inaccuracies that can't really matter that much" add up.

*FWIW, "calorie-counting wristbands" vary quite a bit in how they do things. Some of them will tell you the extra calories that you burned doing exercise, but yours sounds like one that tells you the total calories you burned during the exercise period -- that includes your BMR calories that you would have burned anyway. Whatever. Wrist-trackers are not terribly accurate, but generally speaking I agree that they're going to give you lower calorie counts than the database. Which again, suggests to me that you're going for longer than an hour and/or you are a bigger person than you've implied.