Congratualtions an all the pregnancie , but try not to worry too much. Stretching pains are normal early on in the first trimester, as your uterus is stretching and the hormones are softening things up. It's also common for the corpus luteum cyst (it forms on the overy that released the egg and provides all the hormones until the placenta kicks in at 12-14 weeks) to give a bit of pain, especially if it is enlarged, which is why it is often one-sided.
However, other reasons for the pain could be a UTI, which don't always present with pain on weeing like in non-pregnancy, so make sure you are drinking plenty of fluids, especially cranberry juice. Ectopic pregnancies can present with severe shoulder pain on the side of the damaged tube, but having spoken with friends who have had ectopic pregnancies, it is severe pain.
Provided there is no bleeding, and by that I mean fresh red blood rather than brown, old blood which is usually from an implantation bleed, then things are probably OK.
I called the Early pregnancy unit this pregnancy as the pain was horrible and having had two previous MCs was a bit worried. They told me to take some pain killers as if there was no pain the worry would go away . Anyhoo, it's ok to take paracetomol at this stage, so don't feel you have to suffer.
I'm sure everything is well.
Good Luck