Bleeding at low hcg at 4-5 weeks. Any similar experienced?

Got bleeding since yesterday, more of a rusty brown color especially when I peeing. Got ultrasound scan and transvaginal but nothing seems suspicious except the fact that cannot detect the gestational sac due to Im only 4-5 weeks. But my hcg on 25 May was at 321 and today 29 May it's only 481. Did not up as expected (Obgyne said should be atleast 1800-2000). I don't know what to think. They not confirming its miscarriage until I back on 4 June. I had a mild to severe crampings and lots of gas pain. Anyone does experience similar situation and get manage to get a rise on hcg and healthy pregnancy? btw... I'm on Duphaston, progesterone gel and eostrogel.