Grainy discharge

my boyfriend(of two years)and i began having sex about five months ago. we lost our virginity to eachother. about a month ago i was put on the depo birth control shot, before that we used condoms. so i am not supposed to have my next period until late august. i have heard of women's intuition telling them they are pregnant. i started feeling that way about a week ago. about 4 days ago(wednesday) i woke up at around 3 a.m. because my lower abdomen and lower back hurt severely. i also had an odd feeling in my lower belly. i went to the bathroom and when i wiped there was blood and a whitish-purple veiny tissue on the paper(about dimesized)and in the toilet were large dark&bright red blood clots(nickle to quarter sized). at 7 that morning my bf came to pick me up and we did some research and decided that i had miscarried. i didnt bleed anymore that day. there is a light brown grainy discharge that leaves my vagina. what does that mean? we recently had sex in his pool. did that cause that?