I've lost 15lbs in 18 days. Should I be worried?

The first two weeks-ish can be unusual. However, if that kind of loss rate continues, eat more. If you have 50 pounds to lose and weigh 200, you're kind of at the border where 2 pounds a week becomes more aggressive rather than sensibly moderate in the first place. (1% per week, slower as you get within about 50 pounds of goal - that's a common rule of thumb around here.)

If you start to feel fatigued, weak, find yourself doing a bit less in daily life, maybe not getting your usual exercise intensity easily - definitely eat more as soon as you see that happening.

The so-called "calculators", like the one built into MFP or TDEE calculators, just give you calorie estimates that are the average value based on large-scale population studies, and a few key demographic facts about you. Reality is more complicated. While (by definition) most people are close to average, thus close to the calculator results, some people can have different experiences, either high or low. Once you start to see your average weekly weight loss begin to stabilize **, believe that, not the calculators.

** Note to other readers: OP is male. Weekly weight lost is more likely begin to hover around a typical value per week, up a little some weeks, down a little other other weeks depending on exercise, sodium, carbs, etc. For premenopausal women, hormonal factors can make things a little weirder for some, makng it more rational to look at weight loss from one benchmark point in your menstrual cycle to the same point in the next cycle, and averaging that over the intervening weeks.