Symptoms?

My periods ended about over a week ago. And I had unprotected sex the 2nd day after it ended. He did come in me, and iv been having cramping AFTER my period ened. Which is very very rare... Also my back is KILLING me . It's so wierd how bad it hurts and I havnt lifted anything to strain a muscle . My questions are (How many of you ladies had these symptoms & could I have evn conceived right after your period.)