Constipation due to bacterial overgrowth?

I have had constipation for many years and it has gotten progressively worse. I have tried all types of medicine--herbal, homeopathic, and alopathic--but to no avail. From senna to herbal teas, from silacia to plumbum in homeopathy, from miralax to zelnorm in alopathic -- nothing has worked. For some reason, my body builds resistance to all these medicines, they initially seem to work (however moderate the impact may be) but then gradually become ineffective. I have even tried various products commonly found on this forum, including colon cleansers and am now trying out Oxypowder and noticing that while it was quite effective initially, it is losing effect gradually. I am essentially to the point where I simply can not go to the bathroom without taking Oxypowder AND enema together. I've changed my diet to add much more vegetables and avoid fast food etc. and add lots of water but this had no impact.
In spite of all this, I did notice that during the past years, whenever I had a severe fever and was given Antibiotics , it seemed to make my bowels somewhat more regular. Initially, I had to take ciprofloxacin due to a neumonia, my I became regular like never before. Again, recently, I was given levaquin (another antibiotic) for a fever that wasn't getting off, and it seemed to improve the regularity of my bowels (to a lesser degree this time but nevertheless the improvement was evident). This got me thinking that maybe I have some severe overgrowth of some kind of bad bacteria that causes severe constipation and whenever I take Antibiotics , the bacteria is killed for some time and my constipation is reduced (until it grows back up). I guess I'm looking for some answers -- particularly does anyone know if my severe constipation could be due to some bacterial overgrowth? (I've read that bacterial overgrowth syndrome usually results in diarreah, not constipation). If so, are there any solutions to this problem? This is making live very difficult to say the least. Any suggestions would be appreciated.