I knocked my shoulder against a door and the next day definite petechiae appeared in that spot, i don't recall this ever happening before. should i wait to see if they resolve or is this serious?

Can Wait: If no pain and mechanical dysfunction nsaid, local heat will help.

Contusion: An injury like that. Is a contusion and could be associated with both petechial and black and blue discoloration. It takes days for these to clear.

Sounds like a bruise: Probably just hit it in the perfectly wrong way. If you do notice a general increase in the ease of bruising though please see your doctor.

Would wait and see: The proximity of the trauma and appearance of the petechiae as you state most likely tie those 2 events together although i would've expected to see more bruising. However, just as easily you nothing could've shown up 24hrs. Later. Therefore, i would watch it for signs of abnormal or unusual progression. Are you on antiplatelet/anticoagulants? Perhaps, ice on the area for a day or so?