5w1d HCG rising but not doubling

Nicole
I'm worried I'm experiencing another ectopic pregnancy. I've had a ruptured ectopic in 2014 resulting in the lost of my right tube. I'm now 5 weeks 1 day and my HCG levels aren't doubling as they should. I don't have any pains or bleeding. Has anyone had low HCG and still had a healthy pregnancy? I'm going in tomorrow for an ultrasound. Any advice would be helpful! 
1st HCG- 41
2nd HCG- 115
3rd HCG- 171
4th HCG- 284