What is Addison’s disease?

I'm 50 year old female and till this occurred I was pretty healthy woman.
Few months ago I started to feel constant fatigue, muscle weakness, and low blood pressure. First I thought I was some normal appearance in older people but my daughter made me visit general practitioner and doctor said that he will prescribe me some antibiotics because he suspects on some generalized inflammation.
He also add that if this don’t help me that I will do some hormone testings because it can be also cause by some disease called Addison’s disease. I'm drinking these antibiotics almost a week now and I must say that I don’t feel any progress what so ever. So I was wondering what kind of disease is this Addison’s disease.
Thanks

I'm diagnosed with Addison’s disease almost ten years ago and will tell you some details about it.
Maybe you have heard that above your kidneys there are two small adrenal glands which produce several hormones very important to your body.
Well, Addison's disease occurs when the adrenal glands do not produce enough of the hormone cortisol and, in some cases, the hormone aldosterone. The disease is also called adrenal insufficiency, or hypocortisolism.
This can be very serious condition because Cortisol's most important job is to help the body respond to stress and Aldosterone helps maintain blood pressure and water and salt balance in the body by helping the kidney retain sodium and excrete potassium.
So you see what could happen without these two hormones.
Therapy is substitional. This means that it involves replacing, or substituting, the hormones that the adrenal glands are not making.