Hi there. I have had a lot of hip flexor issues in the past and I still have to baby them. Here is a little info that I hope might help.
There are actually 3 muscles that make up the hip flexor.
The Rectus Femoris is part of the quadriceps and runs the closest to the outside of your leg (just inside to hip bone). This muscle helps with knee extension. If you have pain when extending your knee, then it could be that muscle.
The Illiopsoas is a group of the other 2 muscles. The Illiacus and the Psoas Major. These run from the lower spine and pelvis into the femur. Since you felt the pain towards your crotch, I suspect the Psoas is the one giving you grief.
The first thing you want to do is use ice for any inflammation that may have occurred. Ice 20 minutes on and 20 minutes off when you are able. If you are an ibuprofen taker, this could help with pain. Rest is super important, so do that as often as possible.
Once the pain starts to subside, you can add in some foam roller massage, some heat and some hip flexor stretches. If you have Veena's lessons, she has some great ones.
Here is a link for the foam roller for Hip Flexors: https://www.studioveena.com/lessons/view/4dd5206b-7ea0-4df1-a74b-78ca0ac37250
Here is a link for the Lying Thigh Stretch:
https://www.studioveena.com/lessons/view/4df7637c-c880-4ab2-850c-6f590ac37250
This link is for the deep lunge:
https://www.studioveena.com/lessons/view/4dfca7ca-0fb0-4805-afbb-20310ac37250
Hope this helps!