As you have already done investigations like ECG and lipids
, which are normal , you need not worry about seriousness of disease .
Besides this you can consider tests like stress test
or TMT and Color doppler of heart which will confirm that the problem is mostly due to non cardiac ( other than Heart ) causes .
Stress and anxiety
may also be one of the reason for this type of complaints .
At present you may not need any drugs , but stress and anxiety management will help you .