Most likely from doing too much and over doing it. The baby is still really low in your pelvic and very small. make sure you start to do side and back stretch. I was like that too when i was about 10 weeks, and now i am 20+4 weeks and I have to go to a physc therapist due to baby sitting all weird in my belly. But they suggested to me to do pregnancy yoga and get your sides and back stretched out, not only will it help with the pain in the back but also get you ready for carrying your baby. Good luck hun, hope the pain goes away. You could try some icy hot or a hot bath.
Thank you sooo much.. I'm going to try it out :)