Everything depends on context.
Since you managed to increase your calories and are still seeing weight loss, it is probably a combination of what others have mentioned here-shedding water weight and possibly glycogen–and that your metabolism has finally speeded up because you’re actually fueling it now.
Rules of thumb: If you are feeling good, not really hungry (I’m not talking about cravings for bad foods, I’m talking physical hunger), then faster fat loss is possible and not dangerous if you are starting from a more overweight spot.
If you are dragging energy wise, or can’t sleep, or always sleepy, then fast weight loss is usually a bad thing because you’re not actually feeding your body enough to run its systems and it is trying to tell you that.
If you are feeling ok but ragingly hungry all the time then fast weight loss is bad because you are not feeding your body enough and it is going to start to ratchet down the metabolism’s speed to try to compensate. For some reason people chop calories off at the knees instead of chipping away…chopping them off at the knees is bad practice because there is only so low you can go before you can’t drop any more. There IS a calorie ‘floor’…at least if you have any interest in being healthy and not damaging yourself. That’s why small changes are most effective–get the most progress out of the least change. Once you reach the floor you have nowhere to go and will have to stop the diet and reverse back in calories slowly to rehabilitate your metabolism.
This is the reason “eat less”–while very true–is too general advice. Alternatively if you kept your calories where they were and simply added activity, you would still create a deficit and lose weight, but feel better. Generally speaking it is better to move MORE than to eat LESS (refer again to the calorie floor concept). This is general advice, not for you specifically. Being that you increased your calories and are seeing progress I am not suggesting you change what you are doing. Wait til it stops working, THEN change something.
In other words, once you stop losing weight entirely for 2 weeks, then add activity OR chisel some calories out. Not both at the same time. Try one, wait a week, add the other on top. Get the most out of the least. This takes longer but means less suffering and less change of metabolic damage.
For you, you’re fine right now. The key danger to losing weight super fast comes mostly when either 1) your calories are already significantly low or 2) you are already pretty lean. It is harder to lose fat quickly when you are very lean, this is why physique and figure competitors diet slowly. The leaner you are, the touchier things get–this is why they track everything they do. Obviously, if you are close to the calorie floor its dangerous to lose weight quickly.