I'm having some memory/concentration problems- is it normal as you grow older? my brain doesn't get enough oxygen
anyway I was 4.0 in physics but I'm worried about this
It is not inevitable that memory and/or concentration worsen with age (up to a certain point anyway). You should not normally see any impairment at any age up to 60 at least, probably more like 75 if you are mentally active (how long did Erdős keep working? - you don't need to answer, I know, thank you).
(that does not mean you will have the same motivation as you get older, boredom and overfamiliarity are serious threats to scientific creativity, or creativity of any kind for that matter)