K.,
definitely go see your doc about the dull ache/pain on your side. Ask for an ultrasound.
As far as getting pregnant so soon, I was in your shoes too (new baby 6 weeks old). So, the last two are 13 months apart. I have heard that the risk of getting pregnant so soon after childbirth is that your uterus has not fully healed and therefore is not an 'ideal' environment, and that the placenta can suffer. but on the other hand, they say that the baby will get what it needs, but you will suffer (nutrition wise). Sooooo, I guess if you take your vitamins and eat well, that's about the best you can do, and the rest is in God's hands.
Also, I'm not sure if you are breastfeeding, but I have also read that nature will take care of the baby outside the body, before the baby inside the body, so if you're breastfeeding, you are taking away from the new baby. I'm not an expert and dont pretend to be... and I know women who have successfully breastfed while pregnant. Definitely ask your doctor what he recommends.
Good luck to you, let us know what happens!