I have a sharp pain on my right side of my hip and it hurts to sit up for a long time?

[Mar mar]

I have no idea what is wrong but i dont feel the pain when i stand but when i sit down for almost 10 minutes the pain begins to appear and it continues to hurt me thoughout the day my coach assumed it mightve been a sprain and gave me a specific stretch to try to cure it and i did that for a week it went away but now if i sit for even an hour or more the pain returns and it hurts to sit and i try to lean to the left to maybe help ease the pain but it relieves it for a bit but shoots right back up

whats wrong? should i even be worried about this?

p.s. i run cross country i dont know if that might be the cause of it