Placenta Pregnancy

I just have a question for some mommy’s that have went through this. I go to a specialist and they have noticed my placenta is borderlined almost over my cervix... I am only 17 weeks pregnant.. they will see me again at 22 weeks and if it is over then they said they are putting me on bed rest.. someone help me through this please! I’m trying not to think so negative but I don’t know what to expect or what to think. What causes placenta to move like this? My last pregnancy was nowhere near like this one.