Just another thought it could be the Corpus Luteum which is "a glandular mass which forms in the ovary after fertilization. It produces progesterone which helps to form the placenta and is active for the first 14 weeks of pregnancy". I'm pretty sure this caused me a bit of discomfort and some of my friends had mentioned that they had felt pain on one side in the early stages.

If you get any other symptoms such as bleeding or anything like that or the pain worsens definitely go to the ER.