It is certainly possible that this was due to swimming in the pool, but it is unlikely in and of itself. People who react to chlorine typically have a long history of it and you would likely have had this issue before. Furthermore, unless you only went up to your waste it would be unusual for it to only occur on the legs. Chlorine can cause a rash in individuals with sensitive skin which is essentially an eczematous type of condition, but typically it is a more diffuse rash. On the other hand, you mention you also shaved your legs that day too. I'm guessing that you did this before you went to the pool for aesthetic reasons. If this is the case, then it does make sense that pool water may have specifically targeted your legs. Even the gentlest shaves cause microscopic cuts in the skin. This is usually of no significance, but if you then pour an irritant over such areas (of which pool water contains many including chlorine, bromine, calcium, alkaline pH, etc.) this could certainly make someone who is predisposed to reaction have one. If your rash has persisted, you should have it evaluated by your dermatologist
or primary care physician
, as rashes due to irritants of the kind just described should be fairly self-limited and not last more than a day or two.