Can Petechiae Come And Go? My Infant Is On Blood

Can petechiae come and go? My infant is on blood thinners (lovenox) and has a little rash on his upper chest, no fever or any other symptoms. Not even sure if it's petechiae, just paranoid because he is on blood thinners. It has been on and off since this afternoon. So not sure if petechiae does that (faded and comes back within hours) or if it's just a skin irritant.