Relapses are caused by dormant parasites becoming mature, replicating and moving from their home in liver cells into red blood cells. These red blood cells eventually rupture, and it is this which causes the symptoms of malaria. The symptoms of a relapse will be the same as for a primary infection (swinging high fever, malaise, joint pain etc).
The reason pregnancy makes it worse is because you have more blood volume when you're pregnant, but a relatively lower percentage of that blood is made up of red blood cells, so you can afford to lose fewer of them. You're also more likely to get complications such as Blackwater fever and cerebral malaria, and the high temperatures can damage the growing brain of the foetus.
Having a dormant malaria infection while you're pregnant should not cause you problems as the parasites are nicely curled up inside your liver cells, and will not get to the foetus or cause you symptoms.