What Causes Fatigue, Drowsiness In Summer For An Otherwise Active Female?

I m a 25 year old female. I m short and admittedly I have a high BMI, however I ve been very involved in many activities. I ve been particularly physically active more than usual because I work at a summer program with children. However I ve noticed that since even before this job, I ve been very sleepy during the summer time. I can understand fatigue after a long hot day of running after 4th graders, but on days off and even years before the job, the drowsiness really gets me. If I d stay home, I d constantly take cat naps. I don t experience this in the other seasons. Last year in June I had to stay in the hospital for a week due to gallstones, and in my stay I was given potassium because supposedly my blood pressure would drop. Could the summer really affect my blood pressure and make me drowsy? Or was that a one time occurrence because of the gallstone incident? If so, what is making me so drowsy in the summer?