Hi and welcome to HCM
You must have injured the ischium
and might be hamstring
insertion which is causing pain while doin splits and sitting as during splits your hamstrings and iliopsoas
Hamstring have their attachment on ischium. What you can do now is go for a clinical check up and a x ray
coccyx lateral view and hip ap view. Then if all ok you can opt for physical modality for 2 weeks and then start exercise for hamstring stretching under supervision.
I do hope that you have found something helpful and informative and I will be glad to answer any further query.