Why Does Head Feel Heavy Leading To Dizziness And A Racing Heart In An Athlete?

I m a female athlete, 59 years old, extremely healthy. For two days I have not been able to stand even 5 minutes, upon standing my head immediately feels heavy, and I feel so dizzy that if I don t lie down I ll pass out. I did pass out once and woke on the kitchen floor vomiting a little. Once I lie down again I usually yawn. No fever . Some hot and cold spells. Feel almost normal lying down or sitting, but can t stand. And if I walk up a flight of stairs I am immediately winded, heart is racing, and all muscles feel like they have lactic acid buildup.