Maisy, your hormones could be off due to the birth of your son, but I'd be willing to bet that the miscarriage was more of a chromosomal error...It happens often, and most often so early, that most women didn't even know they were pregnant in the first place. As high as 50% of pregnancies end in miscarriages!
You're not OUT, until a miscarriage has been confirmed. You may just be surprised today that your hcg is rising well, and that it's just early to have high numbers.
If you do have another loss, you could start pressuring your doc to begin checking your progesterone levels, estrogen, thyroid, etc. There are things they can check NOW, if you remain persistent!