Am I losing weight too fast? 70-75 lbs in 4 months.?

I have lost a lot of weight over the last 4 months with a mix of low carb, low calorie, and working out. The first three months I lost 60 lbs, about 20 lbs a month. For a few weeks I wasn't able to work out very much and the weight loss slowed down considerably. The last week I have been going to the gym twice a day and I have started losing again. (about 7-8lbs this week). Some people say this isn't healthy, but it seems obvious that the main reason I am losing the weight is from working out. I have known people on weight watchers or low carb diets come close to these results without working out at all. As for my diet it is mainly low carb, but I am also trying to watch my calories. Kind of a mix between Weight Watchers and Atkins. I did fast for a while, but lost less weight doing that then eating regularly so I stopped. My body is in a state of ketosis from the low carb diet. That is when your body has no carbs to use for energy, so it uses stored fat instead. Since my body is using fat for energy, and I have been working out a lot, It seems like this amount of weight loss is normal - but I am not a doctor. And as for gaining the weight back I really don't think that will happen. I am very dedicated right now, and almost addicted to working out. I plan on making this a lifestyle. I just broke the 200 lb mark today. My goal is 160 and I really wanted to make it by labor day. Given the fact that I am in ketosis and working out two times a day is this amount of weight loss still considered unhealthy? I am not starving myself and I take vitamins every day as well. Thanks in advance for your advice.