False positives on FRER after miscarriage

I had medical management for my miscarriage in November and got my period finally 8 weeks later. Annoyingly am STILL getting positive FRER tests after AF has finished. Called the EPU and they did a blood test today to check HCG and it is just 3! Anything below 5 is classed as negative. Can't believe the FRER is still giving me a positive. Caused me so much confusion. I'm never using one again. Anyone else had this issue?!