Hi Fionas
This is not unusual and not very pleasant. It is usually a blood pressure problem and also related to your bump and the increase number of blood vessels. Your blood volume can increase by as much as 80%. Your heart has to work harder to pump all this blood around your body. Your heart has to push blood uphill to get the oxygen to your brain, as you can imagine this is hard work. If the blood to your brain is reduced for even a second or two you can pass out or feel like you are going to.
So what happens after a shower?
When you have had a shower your body is nice and warm. Unfortunately your body wants to cool it down. It does this by opening, as wide as possible, all the small blood vessels that are close to the surface of the skin. Now usually a lot of these blood vessels are usually only partly open. When these vessels open a lot more blood rushes into them leaving the brain short of blood. It only takes a couple of seconds for your heart to increase its workload but it is this couple of seconds that make you feel faint.