Hi there, yes, you should go to your GP, but it's probably nothing to worry about.
I had my DD2 by ELCS this April. My first period post-birth (not lochia) lasted for around 28 days - yes, I had a 4-week period.
I was soooooo freaked out. I went to see my GP who examined my abdomen/uterus, arranged for a blood test and trans-vaginal ultrasound scan to check for retained products (unlikely with am ELCS, but still possible).
I had the blood test but didn't need the scan as I'd stopped bleeding by the time the appointment came.
The blood test showed that I had a low lymphocyte (White blood cell) count. This is common after surgery and/or infection (I didn't have any intention after the birth; maybe my body fought it off which is what knocked out the White cells?)
I don't know if this was anything to do with my long period.
The good news is that, almost 7 months later, my periods have -touches wood- reregulated and lasting about 5-6 days.
HTH!