They would have to do a u/s to rule out a ectopic pregnancy I'm not sure if the pain will only be in one side but it could just be your body getting used to you being pregnant.
Maybe you are not as far along as you think you are. I would go back to your Dr and ask for another bloodtest to check your HCG levels to see if they are going up and also ask for a early u/s to find out what is going on if your levels are going up.

ETA: if you are really worried about the pain then go to the hospital as they will do a u/s or at least get you booked in for one next week.