In addition to the good advices other people have already said, you
may climb up a ladder and check the gutter to see if fallen leaves are
clogging the drain hole to the downsprout. This only takes a few
leaves to clog the drain hole. In a heavy rain, the leaves will slow
down the drain hole just enough that the rain water will overflow the
lowest point of the gutter that is where the downsprout is.

Better yet, use a water hose to fill the gutter and see how/where the
water goes. Water can go in an unexpected way that you cannot predict
in a dry day. You want to channel the water away from the house
foundation. I don't remember what is the recommended distance to
direct the water away from the house foundation. I direct water from
the downsprout at least 5 feet away from my house foundation, and the
corners of my basement stay dry since then (there is exception; but
let don't get into details here).

Jay Chan