If you're losing weight that quickly; then it is not fat you're accumulating but I think it is water weight. The bulking up is probably indicative that your effort on the treadmill, despite you trying to simulate it as being as equivalent to the effort of running outdoors is not equal and usually isn't. Running on a treadmill especially on the incline I think impose more effort for you, so then you expand your glycogen stores to cope with that effort. And that in turn will increase the water weight of your body. Whether you're running on pavement or on a treadmill, your cortisol level will still be high unless you're a super elite athlete that has low RHR and WHR that running at breakneck speed is effortless and less stressful to them. Unfortunately, not all of us run like Usain Bolt and neither do we run like Hailee Grebrselassie. I don't think your gaining muscles so quickly either. For an ideal woman with proper training and nutrition and due to a lower level of testosterone, you will gain about 0.12 to 0.25 lbs per week in muscle growth from strength and resistance training and that is assuming you are in a caloric surplus diet which you are not. And it takes about 1 week at 500 calories surplus/day to gain 1 pound of fat, which I think is quite highly unlikely. You see, in order to gain fat, you need to exceed your glycogen stores and any excess glucose will then turn into fat and you won't easily lose that weight when you go out for a run on pavement that fast as well.
I think you are fixated on the wrong outcome for your run training. Exercise has never meant to be for pure weight loss. It is a companion to weight loss. It is diet; proper nutritional diet with full nutrients, minerals and the right macros that help with good proper weight loss. As soon as you run and your run very hard, you trigger the mechanism in your body to adapt and strengthen so the next time around you will run better and stronger with fewer injuries. That weight increase is temporary and is off-set by losing body fat. Weight loss is not linear; it goes up and down. What's really important is not the scale, but rather your waist line and other areas where fat would accumulate and are they shrinking? If they are; be happy and keep working out. Eventually, you will be at a healthy weight where you won't get such a wild swing.
The wild swing you are getting is that, you are attempting to control your body's growth by caloric restriction and hard exercises; both of which will need a caloric surplus to help your build your muscles mass and bone mass so you can do a greater work load. By increasing work load, you can then burn more fat. But since you are controlling your caloric input and starving the body, you are putting the body out of homeostasis, which then is giving you some feedback that your current approach in diet and exercise need re-evaluating.
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