I would make sure you are getting enough protein in your diet. Along with giving your muscles time to recover and repair, you need to feed them what they need to regenerate. I only wait one day between strength training sessions. I do it on M-W-F for 1/2 hour a day. 90 mins total a week. Are you training for too long I wonder? Anyway, your muscles will adjust as you get more and more defined. Just make sure you are challenging yourself and not just doing the same thing week after week. As you find it easier and easier, you need to kick it up a notch!
Best of luck. I love strength training!!!! It's my savings account in the fitness bank!