Anyone heard/read anything about amphetamines accumulating in the body after time?

No. I've never heard of that. Where did you get that info (source)

I haven't heard anything like that either. In fact, if I understand correctly the reason that amphetamines and other similar stimulants take effect right away is because they DON'T accumulate in the body.

My Doctor mentionned somehing like that

I wonder where the doctor learned it... Really curious.