I started having this within the last year. I'm nearly 30, have never been sexually active or on birth control. I use a diva cup and the clear liquid floats to the top. I dumped my cup into a bowl to see better detail and again, blood on bottom, mirky clear on top. It varies in amount but can sometimes be half of the contents of my cup. Considering it started so late in the lifetime of my cycle, and nothing has changed with vagina action or hormone stuff, it seems weird. I've been using a cup for about 10 years so it's not like that's changed. Maybe some day they will figure it out.
However, after reading through these comments it would appear that it could very well be due to CYSTS and/or FIBROIDS bursting during your period (possibly hormonal fluctuations are tied into it somehow, but who knows). Women of all ages can get cysts/fibroids regardless of whether they're on the pill or not. Some get excruciating pain from cysts and fibroids, while some women can get them and not even know.
All I know is I had a lot of pain yesterday and my period (which would usually be a lot heavier at this time) seemed to become light pink (ended up with a tampon soaking wet with nearly clear fluid), but not really hurting today and barely any clear fluid compared to yesterday, so I definitely think it must've been cysts/fibroids.
I've been the same the last couple of months. Only at the end of my period. It smells like dirty water and I've never experienced it before. I'm almost 40. I'm so self conscious when I'm on and feel dirty. It prompted me to get a smear to see if anything showed up. I'm waiting for results. But when I mentioned it to the nurse she was also puzzled. I'm in a relationship and I'm not on birth control as my partner is infertile. I'm glad I'm not the only one. I thought it was some kind of cancer tbh..