CD already has MANY threads about this.
OP - use the search engine, try terms like memory, you will find a wealth of information, including answers to your questions, which I will briefly answer anyway.
Memory becomes impaired because the hyppocampus is where a ton of CB receptor sites can line, which is what THC binds to - the problem with this is it sort of "overloads" the hyppocampus, which is where short term memory is. When you smoke, THC travels from one synapse down an axon to the end of that link, and releases through your synaptic clefts, which then binds and "floods" your hippocampus area. Memory loss happens because you are high and your hyppocampus isn't working quiet as intended, which is why you might lose your thoughts mid sentence while high, and also why you don't remember your dreams, as CB1 and CB2 receptors also are located in an area that effects sleep cycles, and your short term memory can't remember the even shorter dreaming process. (This is the neuropsychology 101 for simplicity sake, it doesn't work quiet like that, but you get the idea).
Working out, taking memory supplements, or taking a break will help restore your short term memory. The longer the break, the more difference you will notice in your memory. Remember, cannabis is still a drug (psychedelic), albeit barely.
Search, I and others have written a lot about it in the past.
/closed