Several reasons. You can deplete your body with too many high intensity work outs. You should drink enough fluids and give your body the right nutrition. You should eat a balanced meal with high quality protein and carbohydrages within one hour of an intense work out to prevent depletion of your glycogen stores. This will make recovery better between work outs and help with sleep. Avoid caffeine intake late in day.
Lack of recovery. Your system needs proper recovery time. Muscle rest, of course, but chemical recovery as well. You create tremendous amounts of breakdown and by-products when training, and increase these when overtraining. Dehydration is also likely. Urine should be clear or light yellow only, otherwise -- dehydrated. At 45 may need Ibuprofen to decrease inflammation. Rest is key. Sleep 8 - 9 hrs a must.