+ morning upt but negative blood hcg.

I had my periods on 9th September Got intimate on 13th September with protection. Had faint line on test 2 days before periods along with mild right side pain and spotting. Did serum beta hcg and it was negative And tvs at 4 week both indicated not pregnant. Started bleeding on my due date bleeding got heavier on 2nd day and almost stopped on 3rd day hut this period was unusual and doctor indicated it might be chemical pregnancy. My pain and bleeding stopped. Again this month i didn't got intimate and yet i was more than one week late. Did upt yesterday and it was positive, again did upt after few hours and it was negative, did beta hcg and it was negative ( less than 2.0) Today again i did morning upt and it was positive. What can cause position upt but negative beta hcg?