Scary Bleeding, Need Help

I am a 16 y/o girl and I have recently slept with someone new. I had been sleeping with this person for about a 2 1/2 weeks when tonight I felt pain when peeing and saw blood. I'm not sure what to do or what it is and am need advice. Does anyone know how I should handle this? I want to take care of things responsibly but seeing a doctor might be an issue.