What Causes Frequent Episodes Of Nausea, Hot Flashes, Vomiting, And Fainting In Two Children At The Same Time?

My daughter is 16, she has had nausea episodes since she was about 8-9 (anxiety diagnosed) for the last 2 years, she has been having these episodes of hot flashes, vomitting, fainting ( again aniety diagnosed, or mystery) My son is 8 and he started having very similar episodes 9 months are not quiet as sever (hot flash, vomiting s, dizziness (diagnosed with asthma) - My daughter had one of these episodes at the clinic and at the begging of the attack her blood pressure was recorded at 80/50 with her heart rate 51. she was sent to the er where her vitals became normal again and she was released. Both my kids have these episodes at least 5 days a week each if not daily