“We've been going through a period of weak sales.”

Your company's sales numbers have been decreasing for 3 months. You are giving a presentation to some coworkers and want to point out this problem. You say:

We've been going through a period of weak sales for the last few months.

(someone) has been (doing something) for (a length of time)

"___ has been ___ing" is used to show an action or a situation that has continued for some length of time, and is still happening now:

We've been saving up for a trip to Paris.

Have you been working out?

The length of time that the action has continued is expressed with "for ___".

go through (an experience)

This phrase means to experience something that is difficult, unpleasant, or negative. Some examples:

go through a divorce

go through a tough time

going through an economic downturn

go through childbirth

Notice that there is also another meaning of "go through" that's used for objects and lists.

a period of (something)

You can use "period" to describe a specific length of time that has certain characteristics. In the example above, the period is identified as one of "weak sales". Periods can last anywhere from several days to many years.

A few more common examples used with "period" are:

a growth period

a period of transition

a period of war

a period of depression

an artistic period (in a person's life)

weak sales / strong sales

"Sales" are the number of items you sell or the amount of money you make from selling things. Sales can be either "weak" (not much) or "strong" (a lot).

the last (number)(minutes/hours/days/weeks/etc.)

This phrase tells how long something happened up until now. In positive sentences, it means that something continued from the time specified until now:

I was in Boston for the last 3 days.

In negative sentences, it means that something did not happen during that time:

We haven't eaten anything for the last 8 hours.

You can say a specific number, or use a general word like "few" or "several":

She hasn't heard from him in the last several days.