Can petechia start as a solid pink spot and then change into the classic pin prick spots?

Rashes: It sounds like you may be describing a purpuric rash - which is similar to petechiae, except it's a complete 'sheet' of rash... It suggests to me that you have some hematologic issues? If so- I would definitely want to get this checked out by a doctor. They should be checking a complete blood count among other things. Have you had any mosquito bites? I hope you can get a proper evaluation ASAP.