Had a little rough sex yesterday and I also made too much force and I started bleeding. No cramps or pain only when I sit down. Why?

You may be injured. Most likely you sustain an injury to your soft tissues. I think it's a good idea to see your Gyn. The location of injury will determine if you need any special treatment (stitched for sizable laceration) or no treatment at all. Sitz baths with Epsom salt may sooth the irritation in injured area. Additioinally, I'd advise to stay away from inercourse for at least a week.