I've been on antibiotics for 8 days for an ear infection. I still have fluid in my ear and the pain is returning. Should I return to pcp?

Yes. Occasionally fluid behind the eardrum will actually need to be drained. Also, if the fluid is coming from the sinuses and not the ear itself treatment may need to be adjusted. Either way, if your symptoms are returning or worsening, definitely get back in and let them have another look.
Yes. After you have had an ear infection, the fluid can stay around for 3 months or longer. In about 50% of people, it is gone in 1 month, in 90% it is gone in 3 months. If the pain is intense, it may be that the infection is not fully cleared. If it is pain associated with popping and crackling in the ears, but is otherwise not there, then it may just be left over fluid that is not infected.
Yes. When an ear infection clears, steirile fluid may remain. Speak to your doctor to determine if you are experiencing serious otitis media which might be helped with decongetants. Part of the differential diagnosis is TMJ paiin which can present like otits media pain. Your doctor shoud be able to differentiate.