What Causes A Delayed Menstrual Period With Back Pain?

I started my last period during the week of Nov. 27. I had unprotected sex last week. My partner pulled out but did not ejaculate. He went back inside of me and pulled out when he orgasmed. He ejaculated on my pubic hair and I think that some of it dripped down my inner thigh. I am not sure if some got into my vagina or not. I wiped myself with tissue and used hand sanitizer before getting dressed. A week later I do not have any symptoms of getting my period except tiredness, thirst and lower back pain. I have been having lower back pain for a while now. I usually have breast tenderness. I am on day 32 now and am freaking out. I also am recovering from a mild stomach virus that I caught on Sunday. I was vomiting and had very bad diarrhea. Could I be pregnant? Please help!!!! Thanks.