Hi, I Realize That I Sometimes Get Goose Bumps When

Hi, I realize that I sometimes get goose bumps when walking or standing in the sun. Sometimes not only in the sun, but when I'm in shade. I am a student and sometimes when I walk in the sun I get goosebumps. Some times I get anxious or nervous when I'm suppose to make a class presentation. I'm not sure if the goosebumps are related to my "anxiety" or my period as i sometime get goosebumps near, or during or after seeing it. What exactly could be the cause of getting goosebumps in the hot sun?