So really concerned about my body, I need some help if you could give some ideas to what it could be that would be a huge help.

So I went and got a blood test done bc the pregnancy test I took was invalid and it was negative then that day I got the results I started to bleed dark blood and I was like ok I’m getting my period so I put in a tampon, well I noticed that I wasn’t supposed to start my period until the 21st and that was in the 17th . Later that day I took out that tampon and there was barley anything on it. So I kept out the tampon I haven’t bled since. Me and my fiancée are trying for another, so we had sex on and the day before my ovulation. Any ideas of what it could be?