Lmp aug 14 for 4 days. last intercourse aug 12. period was unusual. pain in rectum and vagina. diagnosed pid. could this be mistaken for pregnancy?

Probably not: With your last period being unusual for you it is possible you are pregnant but not very likely. However you can check a pregnancy test to be sure. It is common for a woman with pid to have unusual bleeding and more pain than usual. If it is pid then the pain should start improving as you take antibiotics.

PID vs Pregnancy: So your doctor diagnosed you with pid and likely started you on oral antibiotics. In the course of this workup, I am hoping that they did a pregnancy test. If the test is negative, then this is not the issue. In answer to your question, yes a patient could be assumed to be pregnant when pid was actually the issue. This would be sorted out with a pregnancy test, an exam, and an ultrasound!