1. General deconditioning and as the day wears on you grow more fatigued.
2. Myesthenia gravis, an autoimmune muscle disease where muscle become more fatigued with use.
3. Multiple sclerosis (MS). MS is especially aggravated by fatigue and heat.
I certainly recommend you work closely with your doctor to determine the cause of your weakness, so the two of you can then formulate a plan to treat it.