This is Nature's way of getting you to focus on the child you are carrying or has just been born instead of creating another one. Since you have had two children in five years of marriage, this indicates you had another child soon after the first. The tiredness alone of adapting to a new baby can lower your sex drive, and then when the second came along, you had the first to care for too. Time is the biggest difficulty in regard to sex drive for most couples. Remember there are two pathways to arousal, the autogenic and the psychogenic.
Some people walk around with a lot of physical sex drive.
Others of us have to be intentional about it. That is, we have to psyche ourselves up, put ourselves in the mood. We have to think about it, remind ourselves that it is important. You have to make time for sex. If sex is important to your relationship then you have to make it a priority.