I have a lost tooth filling. didn't get it treated for at least 3 months now. can this be the cause of headaches and head pressure thinking sinuses?

Possibly...: The filling may have fallen out due to recurrent decay. The deep decay may have caused the nerve in the tooth to die. Now, the inflammation/infection may be causing pressure in the bone. This is all speculation, and without seeing a local dentist you will not be sure. Make a call for an appointment. The tooth needs to be restored anyway...

Various Symptoms: See a dentist for a dental evaluation and proper dental care in a timely fashion. Also see a physician for a medical evaluation. Not seeking routine care and especially when you know there's a problem, such as a lost filling, will always result in bigger problems, more grief and larger expenses.