You weight should also be determined by your frame size, along with your height. If you have a large frame, your healthy weight would be higher than if your frame is very small.
Here's a portion of a height/weight chart that might be helpful.
Height Small Frame Medium Frame Large Frame
5'4" 114-127 124-138 134-151
If you have a small frame, you'd be okay at 120, but underweight at 110; likewise, if you have a medium frame, you would be underweight, even at 120.
Thank you so much... I was really worried about the milk supply... And I hope that in about 2 months I'll be around 120lb... Thank you again. :)
While 120lbs is still healthy for your age and frame size, you'll want to compare this to your pre-baby weight. Hormonal changes that happen during the pregnancy make it extremely difficult to achieve your pre-baby weight. As far as the speed of weight loss, 1-2lbs per week is a healthy rate of weight loss - especially for someone like you that isn't too far from "ideal body weight".
There isn't anything magical about Insanity or other similar programs. They are high intensity exercises that can be done at home with little equipment - really nothing more to it than that. But, many people find these exercise routines helpful and have seen results from them. As long as you are healthy (no cardiovascular conditions, diabetes, or other known diseases), performing high intensity exercises shouldn't be a problem.
Oh, and don't worry about your milk supply - you and your baby will be just fine!