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The blacking out that you described is also known as syncope. You say "to the point" so I understand that you "almost" black out, but not quite, which would be called near-syncope or threatened-syncope.
This is caused by insufficient blood flow to the brain. More specifically, it is when the brain does not recieve enough oxygen. When the brain doesn't get enough oxygen, it is thrown into a sort of panic mode and it shuts down, causing you to lose conciousness, or in your case to feel dizzy. This is your brain's way of saying "hold on, I need some air!"
The most likely cause for this is the manner in which you are laughing. You are probably laughing in a way that you are not breathing adequately. This may be due to "silent laughing" where you are actually holding your breath, or you may be breathing too shallowly and rapidly for adequate oxygen exchange to occur in your lungs.
The best thing to do is to be aware of this as a problem. Next time you laugh, practice being aware of how you are breathing. You may need to force yourself to take a few slow deep breaths. This may be difficult because the manner in which you laugh is a deep-seeded habitual behavior. However, if you are aware of your need to breath and you can begin practicing slower, more effective breathing, this will eventually become your new habit and laughing will be more enjoyable.
Oh no, I'm sorry to hear that you completely blacked out once. That must have been miserable, especially being that it was the result of laughing, which should only be a happy thing.
You are correct in thinking that gaining a considerable amount of weight in a small amount of time may contribute to this. Any weight that aids in restricting your lungs from expanding fully will also aid in your ineffective tissue perfusion; that is, it will keep the oxygen from reaching your brain as effectively.
Losing the weight in an appropriate and realistic timeline will help you to fix this problem. The exercising that you will do while you lose the weight will also build up your endurance and increase lung expansion, helping you breath better in the long run!
I hate to tell you to change the way that you laugh because it is such an endearing part of each person's personality; but because this is a threat to your health and safety (you are at risk for injury if you pass out!), you may also have to remind yourself not to bend over and compress your diaphragm while you laugh. That is unfortunate because laughing hard is a great joy in life, but you must think safety first and take care of yourself!