Firstly, is sounds as if you possibly have a generalised anxiety disorder.
Secondly- your mind is not 'messing' with you. It's a part of you. Sometimes it can do things you don't necessarily want or wish it to do- but nevertheless, it's not intentionally working against you.
You have asthma, so it's possible that this is relational cause of your anxiety attacks. It could be that you are subconsciously anticipating an unexpected asthma attack & this is subsequently bringing on a panic attack. The reason I've mentioned about the 'generalised' part of your anxiety is that you haven't mentioned a specific situation in which you have the panic attacks (for ex; around people, when you are stressed, when you are alone, etc).
If you breathing has been checked & is fine, it does sound like you are experiencing the psychological symptoms of anxiety, which are in turn impacting on the way your body reacts to your thoughts.
Go back to your doctors and mention how you feel. He/she may be able to recommend something- not necessarily medication given your age. Or if you have a school counsellor/ pastorial worker, possibly you could have a chat with them.
What else is going on in your life that is making you feel anxious? There may be other causes, such as hormones & peer pressure/school pressure/family pressures, that you are repressing and subsequently, coming out in periods of anxiety.
I wouldn't worry to much about the pulse underneath your stomach when you are lying down- although, if you are really worried or experiencing some pain- always see your doctor & get it checked out.
The dimensions of our body change when we lie down and our organs are pushed forward by the force of gravity (effectively) pulling us & keeping us on a surface. The 'pulse' you see is probably a real pulse or signs of gas bubbles going through your system (depending on your size, place of pulse, etc).
I really hope you are feeling better soon. A few tips for when you have an anxiety attack: stay focused on a mundane thought, something like a maths equation/list of homework tasks or chores; think forward- try and keep your thoughts on future events & not the present moment. If you can- get out & take a few steady deep breaths of air- clear your head away from whatever set you off. Again, in this time- bring your thoughts on to something bigger.
An old panic attack strategy of mine is to think of space. I LOVE astro- science (sub-atomic particles, star formation, time theories, etc) & I LOVE evolution theories, so I try and focus on them. I also count things around me- glasses on a table, lamp-posts, blonde haired people- whatever I can to organise my mind & slow down my heart beat.
Adjusting your thoughts will in turn adjust your physical reactions.
Good luck & I hope you have some relief soon.