My Daughter Who Is 22 Has Had These Sudden Attacks/episodes

My daughter who is 22 has had these sudden attacks/episodes over the past 1 1/2 years. They last about 20 min where her chest by her diaphragm gets really tight, sharp severe pain, and has immense trouble breathing. She's been to the emergency room and doctor. They come on unexpectedly. She just had one bad attack last night, and two today which is unusual. Thought maybe a food allergy triggers it, but this last time, she hadn't eaten anything spicy and had not even eaten anything at all prior to attack. She has had 6 episodes the past 1 1/2 years. Thought maybe gall bladder? but they kind of ruled that out. Did blood work. Nothing. No one really knows, but she is currently pregnant, so very concerned that she just had 3 attacks in the last 2 days. What could this be??