If I have only had one partner, and did not have an std, how could I have pid?

You DID have an STD. Simple answer, you did have an std and still do. Likely gonorrhea and/or chlamydia. For both infections, it's not uncommon to be infected, but not have obvious symptoms. That is why testing often is so important!
Misinformed? I would hate to be the one to break it to you, but you had or have gonorrhea or chlamydia. If you only had one sex partner then yes he did have it to pass along to you. Many with gc or chlamydia are asymptomatic but are still highly contagious.