Hi hepcat:
i just have a couple questions for you... Are you male or female? Sorry for the silly question, but it makes a difference in the way your body will react to exercise and build muscle. I would definitely try to gain a little bit of weight . You are quite a bit under your healthy body mass index ( number devised by a calculation using your height and weight). For someone your heighth, you would want to weigh between 135 and 170 to be considered in a normal weight range. So maybe try to gain about 10 pounds. In doing so, the healthiest way would be to eat food with lots of nutrition, protein will be key to building muscle. But whatever you do, don't cut back on food at all especially if you have started exercising more than you used to since you will be burning more calories now.
Definitely take a day off between working muscle groups. When you work out hard and you get a little sore, you are actually creating tiny tears in your muscle. Then the muscle repairs itself in the next couple days and gets stronger. But if you don't give it time to repair, you will just do more damage. If you want to work out 5 days a week or more, alternate working body parts on different days. I work out all 5 days but I do certain muscles on certain days in a way that I have a balanced workout with all the mu.Scles in my body getting the work out that they need each week. If you look around on the internet a bit, some websites cangive you some good ideas of how to do this.
Let me know if you have any more questions. Try to gain a little bit of weight!! :)