Yes it's possible to exercise too much, but you aren't there yet.
Walking is great, but it isn't taxing your system. As you lose weight, you move less weight while walking and therefore burn less calories. When you hit a standstill, you either need to eat less *boooo* or burn more by upping the intensity. Our bodies adapt quickly to the demands we place on them, so shake it up by swimming or weight lifting or running. The spinning 3x a week is great...it's a huge calorie burner.
Four pounds in a month is perfect, btw. One pound a week is great progress, one I wish I was replicating!
I'm a huge advocate of rest days, depending on what your workouts consist of. Walking daily is great, I dont think anyone should take a break from that. Weight training should be done 2-3x a week at most, since gains are made while the muscle repairs itself. Intense cardio (meaning you hit 80% of your max HR or higher) CAN be daily, but I prefer one or two days without.
You'll know if you are doing too much. Your body will tell you and it won't be subtle about it. Aching, weak, tired muscles. Lack of motivation. Irritability and depression. And the most telling? A resting heart rate that is higher than normal. If you start feeling symptoms like that you can decrease the intensity of your workouts, or suspend them completely for 3-5 days to let your body get the rest it needs.
Personally, I'd start hitting the weights now. Build muscle. It's more metabolically active than fat, so it requires more energy *calories* to maintain. Or do some body weight exercises. Pushups, dips, squats and lunges etc. You MAY gain a few pounds intially, but you'll lean out quicker in the long run.
You are totally kicking butt, btw. And look gorgeous!
