Ibuprofen might reduce the number of some types of bacteria - and if so, maybe whatever bacteria that is happens to also be reduced in numbers by that Antibiotic you took previously. Yet neither the Antibiotic nor the ibuprofen kills all of that bacteria, thus they remultiply when the Antibiotic or ibuprofen is discontinued...just a guess? I know that aspirin will kill some types of bacteria in petri dishes, (I read some study about that), and I have also read that Ibuprofen kills yeast specifically (candida), but I have not yet seen a study showing ibuprofen to kill bacteria...but I would not be surprised if it did, to some degree.
I'm not a regular on this forum, just stumbled on your post in my search for back strain info, but thought I'd put this idea out there for you in case it helps.