Someone most likely used your bedroom for sex; perhaps it was your son. If so, your son knows that you'll notice the blood on the sheet. So the whole family has been living with heightened anxiety. Part of the natural consequences of his misdeeds was having you discover these things and ask him about them.

Calmly explain what you found and ask him to tell you what happened in your bedroom. I'd also have a discussion with him about his sexual activity with his girlfriend. It will be difficult to hold back your anger. You son has made some serious mistakes in judgment and has shaken your trust in him. Let him know these feelings and your profound disappointment in his actions. Allow him to redeem himself with honesty, remorse, and a heartfelt promise to behave with more integrity. Ask yourselves, "What do we want most to come out of this talk with our son?" When you have your answer to that question, you will know how best to proceed.