HCG still rising

I started bleeding at 5 weeks and assumed I had miscarried as the bleeding was heavy with lots of clots. I went to see my doctor who agreed that it was likely to be a miscarriage but wanted to check my HCG levels. I had a blood test last Wednesday and my HCG was 280. By Friday that week it was 307 so not enough to be a viable pregnancy. They then did a scan on Saturday and couldn't see anything in the uterus or anything in my tubes. I had another blood test this Wednesday and assumed I would see the level drop but it's now 387. I'm just getting really worried that I'm now going to have to have some kind of procedure. Has anyone else been through this?