At 21, you shouldn't be affected by this! But with 16 college credits, the homework involved, your part-time job and any social life at all, you may be sleep-deprived and a little nutrition-deprived. You are busy, going from place to place, keeping up with assignments and friends and work schedules.
But I might ask, also, if you have ever been tested for ADD? Attention Deficit Disorder? Only because we (I have ADD) can be very forgetful at times, since our mind is already busy doing 100 other things. We are so prone to distraction that a "typical" thing like someone saying "hi" to us can just go by the wayside pretty quickly.
How is your diet? Do you get enough lean protein? Fruits and veggies?
Do you get any regular exercise?
And do you get enough sleep?