I'm the exact same way. I get very irritable when I'm hot and it's humid outside. I love sweating when I'm working out, but if I'm gonna sweat from just standing outside, hell no. I'm much happier when it's a tad chillier out. I was also born right around Christmas.
Throughout primary/secondary school, the transition into summer always brought with it the end of basketball season and right into soccer conditioning. I loved both sports, but soccer became the source of a lot of stress for me when we got a new coach.
So maybe this underlying disturbance isn't because of one traumatic event but just the same, smaller events that would happen every year around now. Thank you for sharing.