I think most of your symptoms are related to poor exercise condition and anxiety.
Make sure you drink enough, sleep enough and don't eat heavy meals before exercise. Walking 3 km is not considered heavy effort for someone at your age, but it's a great start.
I know you have a sneaking suspicion that you have angina. I won't even comment that. I don't think anyone at age 18 in the world ever had angina.
If you can't get reassured by this, I think you should ask your doctor for a stress test. But you really need to make a promise to yourself in front of that. If (when) you find out that the test is normal, you need to let this issue go. I'm seriously concerned about your anxiety for heart diseases. Nothing in your multiple posts even remotely suggest any heart disease, but your anxiety is not healthy and should be treated.
Thank u. But what about uneasiness in head and jaw which disappeared after rest???
I think it all depends on your overall condition also. Your max hr during exercise is calculated something like 220 - age = min/max hr's (220 - 18) = 202 x min 0.65 and 0.85 max = 131 - 171HR for someone the age of 18.
This is from one of the dr's in the expert forum:
"I would just be able to assume that if you can get to 85% of your maximal predicted heart rate and at the same time exercise at least 8 METS, you have a good functional capacity and are able to do an good amount of exercise without any symptoms." The issue is that, if you are deconditioned or not in good shape your heart rate will jump up very quickly with little exercise and then you would say, oh I have reached goal heart rate but you really haven't exercise very many METS. Whereas, if it takes your heart rate longer to get up to 85% and you exercise 8-9 METS that is great and you are getting into better shape!!!"
here's some great info on METs - http://www.americanheart.org/downloadable/heart/1176406256764PA_Intensity_table_2_1.pdf
A HR of 150 in a young person after walking 3KM seems a bit high, but not dangerous. Here I assume walking, not jogging, and mostly level ground, not a steep hill. That said, the other observations strike me similarly - you may need more regular exercise to build up more endurance. I am 71 and I could walk 3 KM in under 30 minutes (I think I have my metric measures correct here) on the level and not have a HR over 135.
Keep up the exercise is my suggestions....then see what happens in a few weeks.