Your condition sounds like exactly what the physios are trained to treat.
From the sound of it, hyperextension of your back is directly related to worsening of symptoms. So naturally you should avoid it. At least until you get examined by a physiotherapist. The longer you have low back pain, the more likely that it will stay with you, forever... (not kidding) .. so you really need to treat it seriously. GO SEE A PHYSIOTHERAPIST.
In the long run, you should get a GOOD trainer to look at your posture when you are doing your lifts at the gym. From first hand experience, I think you are only ought to maintain your natural curve of your spine when doing weightlifting. Hyperextension of the back causes more weight loading directly onto the facet joints of the spine, and that maybe where your problem is. To avoid repeated exacerbation, you must put more attention to your back.