You are experiencing "over-training." You started off way too fast and too frequently. The main problem is your body's capacity to recover, in particular the central nervous system.
The soreness that you feel is simply called DOMS, or delayed onset muscle soreness, which is caused by the microtears that occurs to muscle when exposed to stress (in this case strenght training). If you feel lethargic, it's the former (over training).
There are several methods to overcome this.
The first is to make a split, such as the one that I do.
Monday - Arms
Tuesday - Back
Thursday - Legs and Abs
Friday - Chest
It's important that you make a split like this for two reasons. 1 is that you cover all the muscles in your body by doing a split like this (for example calves is with legs, forearms with arms, shoulders with back etc), and secondly is that you give your body ample recovery time.
One thing is should be clear though, that on that particular day, you're training that muscle until you can't move. For example, on Monday I did an arm workout with one of my clients and our arms felt like they were going to explode. Now they're sore, but I get to recover them for a week.
On the other side of the coin is nutrition and sleep.
Sleep is simple, you should sleep 8 hours a night.
Nutrition is a bit more complicated and very important.
Depending on what your goals are (probably lose weight and tone up) your caloric intake should only be 200-300 calories below your maintenance intake. If you cut food too much you get that loss of energy you're experience. If you follow this closely and have a proper training program you won't get noticably weaker when losing weight.
Also there's a difference between eating less, and eating good food.
You could stuff your face with healthy food and still lose weight or eat Mc Donald's twice a day and get fat. So dieting, at least to me and my clients, never means starving.
