I am new to open water swimming. Today I went swimming in open water area that was ~59F water temperature (no wetsuit, just swim cap). The first few minutes I felt shock and was very cold. Once I started swimming out (after ~5-10 mins in the water) I stopped feeling the cold and actually started feeling very warm. At one point I felt so warm I wanted to take off my swim cap. The remainder of the swim I felt very warm the whole time. I was very surprised by this. I was in the water for 30 minutes.

Upon exiting the water I felt neither warm nor cold in the 65 F air temperature. I later went in the sauna and also neither felt warm nor cold in the hot sauna. Some of the ladies at the swimming spot mentioned that feeling warm while in cold water can be a sign of hypothermia. Is it possible I was starting to feel hypothermia? Is it possible I was actually overheating in 59F water from vigorous swimming? Is this a unique experience or is it common to feel warm while swimming in cold open water? The sun was also shining. Air temp was at 65F. Same thing happened to me last time I swam in this spot. TIA for any insight.