Pressure/stretching feeling

Megan
I'm 20 weeks, 1 day and started having this pressure/stretching feeling VERY low! It doesn't seem like contractions - It's like a dull ache. I noticed it started when I was walking and it was still uncomfortable when I sat down. It lasted for about 10 minutes. My doctor said it could just be the baby switching positions. Has anyone else had this?!!!!