Can birth control cause false positives?

I had a baby 7 mo the ago and switched from the mini pill used during breast feeding to trisprintec which made me feel depressed so I made an apt to get the mirena and stopped the bc. During that small window I tested positive totally unexpectedly. I called the doctors office today to explain and the lady said the dr wasn’t in today but she would have the nurse call me tomorrow to schedule a blood test but that the influx of hormones from birth control can cause false positives sometimes???????? Tf...I thought hcg was literally produced from having a placenta inside you??? Is that real?