The size of your breasts will have a bearing on whether or not they are "perky". Larger breasts will tend to sag a more than smaller ones due to the extra weight.
There may be exercises you can do to strengthen the muscles, as well. You might do a google search to find some. Some yoga poses might help.
any change of weight recently, don't know if that would cause it, mine got bigger when i gained weight, just an idea i don't know you so it's hard to say, or hormones?