I had cramps there. There are two things it could be: 1 is the baby sitting on a nerve and/or blocking some of the blood flow to your legs. 2 is cramps, for which I was advised to sprinkle extra salt on my food and have a magnesium supplement if it continued. Some people get it for ages but for me I had it on and off from about 20-26 weeks through to the end.
Funny story - once I started kicking my foot in the middle of the night to get things moving and stop the cramp. My DH thought I was having a bad dream and he rolled over and put his arm over me and started saying 'it's alright' etc. I tore his arm off me and fled the bed asap so I could stand up and get the cramp to go away. I was saying 'it's NOT alright!' LOL as I tried to pry his tired-heavy arm off me