Bleeding with Solid Blood Tissues after sex? Why this happened? Can any one help?

We can't possibly know - you got your period, or you were already pregnant and miscarried that day, are two possibilities. Menstrual flow can be quite thin one month and quite full of 'solid tissues' as you put it another month. That's the menstrual lining, or the menstrual lining growing, with a possible embryo the size of a pea that you don't notice. There are countless early miscarriages.

My last periods finished 15 days ago.