This can be the consequence of insufficient glute activity. The glutes and hamstrings have similar jobs, but when the glutes aren't up to par the hamstrings have to work overtime to compensate.
Get those glutes working properly! Single leg glute bridges, pull throughs, basically every leg exercise you do you should put a big emphasis on finishing the movement with your ass.
As far as sitting down, it may be weird, but a ring or donut pillow so the tender spot doesn't touch a chair can help alleviate the pain helping the inflammation to subside.
Hope this helps, let me know if you have any other questions.