I had a similar situation after DS birth. Needed manual removal of placenta, I bleed for around 4 weeks then it went to a light brown discharge/bleed. Went back to the hospital after 6w then 8w they checked me over but were not concerned as it was such a small amount.
Anyway at 12 weeks it was still hanging around and becoming very annoying I was still wearing panty liners/light pads (and I didn't feel right IYKWIM), I went to GP who organised an US. The US showed up retained placenta and I went in for D&C the next day. The private Gyno/OB who did the D&C was very concerned it had been left for so long and told me I was lucky as i could have had a bad outcome.

I would suggest getting a US to be 100% sure. I had to really push for someone to listen to me, but glad I did (to bad it took me so long).

Good Luck, i hope you can get it sorted sooner rather than later!!