Three cycles, spotting 4 days b4 period. Every two wks. Last cycle only spotting for 10 Ultrasound found fluid in c-section scar. What can this mean?

Unstable lining. more details are needed and this is a concern that is probably best managed over a formal consultation, but the fluid probably represents an area that is still healing and unable to create stable endometrial lining, that periodically bleeds given that it is not well developed. give it some time, I would definitely avoid pregnancy during this time and so consider birth control.