Breathlessness at night

im a 17 year old student and recently, i've experienced breathing problems at night. i tend to get short of breath and have very short, shallow breaths. when im midway through my homework i'll feel deprived of air and end up having to do breathing exercises. usually it helps, but recently, for the past few weeks, it hasnt. im fine and breathing properly in the morning though.

i went to the clinic and saw a doctor who found no problems with my lungs and heart. he attributed it to stress and prescribed me with ativan 0.5mg to take when necessary. however, i personally do not feel stressed as everything thing is going well and fine in school and im coping with the heavy workload. i sleep through the night peacefully and wake up feeling refreshed.

what is the problem?