I have come out of hospital and been diagnosed with severe pid. i have an 8 mth old and have not had sex for over a year but am being told it is definitely an sti. how is this possible?

Hard to say: PID is a collection of physical problems usually triggered by a few common STD's. Some like Chlamydia can remain without symptoms in both males & females for extended periods & would only be detected by direct investigation. A low grade infection without obvious symptoms could emerge later with PID. I would ask the treatment team for an explanation since they have the records/testing.