It is still entirely possible that you pulled a muscle in your groin. Sometimes this can happen unexpectedly, for example while you are sleeping, even if you are not aware of it. Nevertheless, other possibilities should be considered.
For example, do you have any swollen spots in the groin? Enlarged lymph nodes in the groin region can sometimes give a feeling of soreness, and they may result from an infection in the genitals or in the leg.
Similarly, does the skin in the groin look red or irritated or does it itchy? If so, then it is possible that you have a fungal infection (jock itch) in the groin that may need to be treated. If you had a very red and warm area of skin, that might be a sign of a more serious skin infection that would need to be investigated immediately by your doctor
I would suggest that you make an appointment to see your primary care doctor
, especially if symptoms worsen or persist. It is a little difficult to tell without a close physical examination what might be going on, but your doctor should be able to help you narrow down the possibilities and suggest a treatment if necessary.