HCG not doubling

I’ve had some spotting and my doctor asked for HCG tests every 48h since last Wednesday. The first test at 4w3d was 1,600. The second one was 3,800, so great news. But then I spotted again and doubling time started slowing down significantly (5,700 48h later, and then 9,000 today). She asked me to test one more time and get an ultrasound next week. Should I be very concerned?