is known to be toxic to kidney
in very high doses for prolonged duration and impairs its function. so it could have decreased your urine output , not increased it.
as regards cephelxin, theirs no scientific study to link the output of urine to that of cephalexin
so its not drug related. but it can be due to the fact that while having these drugs three time a day, you are using water to ingest them. so this water three times a day is the real culprit here. so no need to worry. its perfectly normal.