My gf bleeding after sex?

my gf was bleeding after we had sex and i didnt do it roughly. she was on the depot shot all her life so she has never had a period before. and it only does the spotting but 2 months ago doc told her she been on the shot too long so put her on the daily pills but we havent got it filled yet so she isnt taking anything yet and we had sex she started to bleed but it wasnt a spotting it was alotttt of blood and lasted for awhile she took a shower and it was still bleeding but by the next day its gone... periods usualy last longer... and she had the cramps and pain and sickness the day before this happened. but the blood was alottttt and it was dark colored and it was slimmy feeling like mucis sorta.. but it was blood.. what causes this? she has been to the doc last week they said she as trich bacterial desease butt thats the only one no other std or anything we had afull check done for every desease cause we are wanting to have a baby so that requires to be clean and unprotected sex. we put this on a pause to figure out wat this could be first. her doc says ciould be the body trying to get back into its menstrual cycle.