I had something similar awhile back.  "Mars" my lapper was acting like she didn't feel good, was unsure if it was depression from losing our Tom or if she was generally sick.  I noticed yellow poop so I knew something was wrong started searching and found the worst cases of blackhead, I found the cayenne treatment on this group so figured it was worth a try Mars hated me for a few days but it seemed to help.  She'd take a drink of water with the cayenne shake her head and look at me same with instant mashed potatoes but she improved and eventually recovered.  She still hasn't laid any eggs since but she's too much of a friend to care about that.  It's amazing what these birds can end up meaning to you.