I dont get whats going on

I had a miscarriage in July, my ob cleared me to try an conceive again. This week wednesday i had some light pink spotting an pain on my right side. My last menstrual was september 14th an my period isnt due until October 12th somehow i have had two positive pregnancy test already. The bleeding is on an off now. Went to the er they said my hcg is at 500 but most likely im going to miscarry again but its too soon to tell. I have asked how can i get two positive pregnancy tests so early and no one has answered me. Has this every happened to anyone before? smh im so confused and stressed.