i haven't personally experienced it with any of my kids, but i've researched it a lot (because my middle child gets really high fevers - in the 105 range), and there is a lot of conflicting information out there. some say it's caused by the height of the fever, some say it's caused by the sudden spike in temp, some say that there are certain kids who are just prone to it.... but the one thing they all seem to agree on, is that it looks a lot scarier than it really is. i can't imagine how terrifying it must have been to watch your three month old go through that!!!
anyway, my only advice to you is to find out as much as you can about it, and make an appointment with your baby's doctor to sit down and talk about it too. have a list of questions prepared in advance, and don't let the doctor leave until each question has been answered.
saying lots of prayers for you and your little one