Chest tightness and discomfort on laying down inspite on treatment for Allergic Rhinitis and Asthma when major reports are normal has multiple possibilities. But you need to get evaluated for 2 major things. 1. Muscle spasm at the back of neck or upper back with nerve entrapment leading to chest tightness. 2. Acidity leading to reflux and chest tightness.
Next Steps
Visit your doctor again and ask specifically for these 2 illness, whether you have one. Unless specified by your doctor, there is no sense in doing unnecessary tests.