Matabolism is simple:
Weight Gain: Calories consumed > Calories burned
Weight Loss: Calories consumed < Calories burned
Therefore, if you're losing weight, you're eating fewer calories than you're burning. The solution, eat more calories.
There are other short term considerations that might explain weight loss, such as losing water weight, increased waste elimination, etc, but these are not true weight loss just like emptying oil out of your bike doesn't make the bike truly lighter b/c as soon as you replace the oil, that weight is back.
The key to monitoring your weight is to weigh in at the same time every day, a time when your weight is most stable. A good suggestion is to weigh yourself in the morning right out of bed, before eating and after visiting the toilet. This ensures that you're as "empty" of food/fluid weight as you'll be when you hit the scale.
I can tell you that for a 150 lb person, weight can easily fluctuate up or down by 2 or 3 lbs over the course of any given day, depending on the consistency of their food/liquid intake and level of exercise/activity.
In all likelihood, the fact that you've changed your diet and lost weight means that you've simply overestimated your caloric intake or underestimated your caloric needs or some combination thereof.
My suggestions:
1) Get a good scale. I prefer the type that measure not only weight to the 1/2 pound but also body composition (% of body fat). Scales will measure a LOT of different things these days, but the most important thing is that they be consistent so that if you put a 10lb weight on it today and tomorrow, it should report 10 lbs both days. Worst thing you can do is monitor your weight using an incosistent scale.
2) Weigh in every morning, as I described above. Make a log of the results. After a week you'll have a solid baseline of data. Optional but educational: Weigh yourself as often as you can throughout the day for a while - before and after meals, workouts, bathroom visits, etc. You'll be surprised how quickly weight can fluctuate in a given day. I've lost over 5 lbs in two hours of playing sports due to fluid loss...but it means nothing b/c that weight will be back as I replace those fluids and have a meal.
3) Increase those calories. Make those calories good calories from whole foods, not junk food and soda. You'll gain weight either way, but good calories will help you stay healthy as you gain weight, bad calories can seriously compromise your health. I recommend plant based diets. Many of todays top athletes are vegetarians or vegans for the increased energy and endurance the lack of health complications that non-meat based diets provide over meat based diets.
4) Work out. Weight, cardio, stretching, sports. If you don't, you'll be gaining unhealthy weight. Presumably, you're gaining weight for a reason. Whatever that reason, you want to gain muscle, not fat...unless you're looking to float better in water...but i doubt that's the case. lol
5) Consult a physician, trainer, nutritionist. If you're not sure what you're doing, ask for help from those that do. Physician can evaluate your overall health and ensure that your body is up for meeting the challenges you wish to take on. Trainers can help you learn how to train for your goals. Nutritionist can guid your diet choices for your goals.
6) Get plenty of regular sleep. Go to bed at the same time every night. Get a full nights sleep every night. That's about 8 hours for most of us. For some it's as little as 6 hours. If you wake up refreshed and ready to go, that's a good indicator you've gotten enough sleep, and vice versa. If you want to buid your body up, you have to give it the time to do so. The body repairs and builds when it sleeps. When you workout and push your body, you're tearing it down EVER so slightly, which elicits a response from the body to repair and build up so as to be better able to cope with the new activity. If you don't sleep, you're body won't recover as completely or as quickly. Do NOT underestimate the importance of full and proper sleep.
So, to recap: Get a good scale, weigh in once/day every day, log the results and increase caloric intake to gain weight, work out to put on muscle, consult experts for guidance as needed and get plenty of sleep.
FYI: My first attempt at gaining weight using the above basic principles resulted in 20 lbs of additional muscle and virtually no body fat in about 6 weeks. Bear in mind, the younger you are, the easier it will be to add muscle and minimize fat. Results at 20 will be faster and better than results at age 40, but it can be done at any age.
Best of luck!