Pain in lower abdomen and lower back

Hi all, I am almost 17 weeks pregnant and for a week now I am experiencing pain in my lower abdomen and lower back - especially the left side. Some days it is better, some, like today it is worse. I woke up with this pain for the 1st time 1 week ago and it lasted a few hours. Next days it was still there but not so strong. Today the strong pain is back... I went to my doctor a few days ago, she examined me and said everything looks perfectly fine. Which is of course good news... but... I'm still in pain. Have any of you experienced similar pain and could give me some advice?