I Am 61 Years Old And Have Always Worked Out.

I am 61 years old and have always worked out. I hike, bike, yoga, Pilates, lift lighter weights and Barre classes. For the past several months (even longer, but lately more pronounced), during my Barre class, As soon as I get overheated doing the standing part of the class about 30 minutes into the class...and then I start feeling light-headed. Typically I have had to go back to my mat and lie down. Sometimes I could go back to the standing exercises, but I definitely am still a little light headed. I don't get this way when exercising on my mat either sitting or lying down. Today I got so faint-feeling that I stopped, left my class, then vomited and had to lie down in the hallway several times while walking to the locker room. I thought this could be an electro light problem, so for the past few months ( including today) I have been drinking Pedialite. Now I am wondering if this may have something to do with my blood flow....not getting to my brain. I'm very healthy other than this issue. I'm 5' 7" and weigh 137 pounds. I appreciate your help.