(TMI) what causes this?

My period always starts the first few days being ... I can't even find a good word to describe it ... slimey!? It is clear and stretchy. The blood doesn't really mix with it. Does anyone know what is causing this? The only thing I could find online was that it might be caused by high estrogen. I've wanted to ask for a while, but I was nervous about posting something so intimate. Please let me know if you have experienced this type of period or if you know any possible causes.