What Causes Excessive Sweating And Chills?

My daughter is 30 years old, single mom of a 12 year old girl. Works 30 hours a week. for the past 6 months or so, she has episodes of excessive sweating, where sweat is literally pouring off her entire body, alternating very hot, then chills, throwing up, constipation, feeling like she's going to pass out. She has been to the ER and her own PCP who has run all kinds of blood tests and urine tests and all tests come back normal. She's been told it could be anything from a stomach bug to panic attacks but no one is giving us any real answers. Its doesn't happen all the time, but lately it seems to be happening more often.