Negative hpt but hcg level still in blood

Hayle • 5 yr old daughter. 1 👼🏼 1-23-19. Ttc baby #2.

So I had my cd2 ultrasound today so we can start medication for my next

<a href="https://glowing.com/glow-fertility-program">IUI</a>

cycle. I got my prescription and all. About 2 hrs after I got a call from my nurse said to wait to take my meds because my blood work is still showing hcg. Just by my serum. So they are taking levels tomorrow. My period is heavy and red. I’m assuming if I had retained tissue she would’ve seen it today right? My frer showed negative before my period started. I’m trying to not have a tiny hope that there could be another baby lol since it’s so heavy I highly doubt that. What could cause my levels to show positive still if no retained tissue