So the white patch has always been consistent and now recently it has started to turn red on the right side of the tongue?
My initial guess is that its median rhomboid glossitis which is a fungal infection. Some antifungal medication may help with the issue. You may want to ask your dentist for a prescription. It is possible that the antibiotics which you are taking is killing the bacteria in the mouth but since the bacteria is limited in growth, the fungus in your mouth has taken over but it seems to happen a little more rapidly that I would expect but it is possible.
http://i658.photobucket.com/albums/uu303/water09_2009/tongue3021.jpg
sorry again for all the posts i also wanted to say i have ibs and take alot of imodium (immodium) each week also.sure wish they had an edit button on here.thank you dr.
http://i658.photobucket.com/albums/uu303/water09_2009/tongue4022.jpg
The only way to completely rule out cancer is to have a biopsy performed and looked under a microscope. But if your dentist has ruled it out clinically, I would keep an eye on it and try the anti-fungals. If it improves, great. If not, then you may want to consider a biopsy if it changes, gets worse, etc.