I've been having stomach cramping and feeling very tired and I just felt sick

Ok so I'm 15 if that matters and on the 9th of August I had unprotected sex and I usually start around the 10th or 12th sometime around there . for a couple days I've been having stomach cramping and feeling very tired and I just felt sick + when I eat it feels like just a tease like I get more hungry once I eat something it's weird I've never had that feeling before . I believe it was 2 or 3 days ago when I went to the bathroom to see a big spot of dark brownish gooey looking blood and I thought I had got my period so I put a pad on but realized that everytime I went to the bathroom it was hardly any blood but then It started getting heavier , heavy enough to have stained me today in class it is the 20th of August today . I'm wearing a pad and the "blood" or whatever it is still comes out but I read that some women experience heavy implantation , is it possible that I could still be pregnant ? I "feel" pregnant I've never had this feeling before and I've never had nausea when it comes to my period idk what's going on HELP ME !