The reason people exercise, or more specifically weight-lift, is to provide overload on the body which will then lead to physiological adaptations, the results you want. However, an important factor to the process is recovery, the time you are giving your body to regenerate and adapt to the stress you are putting it under. Proper recovery involves an adequate amount of rest and nutrition intake. When these are lacking, but you are partaking in intense training, over-training may occur.
While many people believe more is better, when it comes to training sometimes less is more. When you do not give your body enough time to rest, you are putting yourself at risk of fatigue, illness, and injuries. Also, you will be working and working but the results you wish for will not be there. In fact, the opposite can occur, in which strength and muscle tone decrease due to excessive fatigue.
If you suspect you are over-training yourself then a reevaluation of your workout program needs to take place. Make sure you are allowing your body enough time to rest and supplying it with the proper amounts of nutrition. Patients is key though, because once over-training has occurred it can take several weeks or even months for your body to restore itself.