With only this information I would say you have some blood leakage. If this continues to grow you need to have a physician treat this. Otherwise, if it is blood it will dissipate by itself over time.
If it has been over two weeks since the activity it probably was not the sex itself that directly caused this. There could have been an issue that the activity brought to light or a condom can cut off blood flow if it is not the right size. This could have injured the muscle area below it.
Keep a close eye on it and take action if it swells more.