Heya!
I woke up with it one morning! And it was on the side that i was not laying on. I thought something was wrong, was really close to getting DP to rush me to hosp! I did some research and came up with 'Round Ligament Pain' which is like Alan said, stretching of the ligaments. I did follow it up with the DR and Midwife, and both agreed that it was Round Ligament Pain, and that it would infact get worse, as there is yet to be more stretching to be done, but there is NOTHING to worry about unless it comes with bleeding and there were a few others.
Ive found it get it more with movements that cause me to use my tummy muscles, such as moving positions in bed, sneezing etc.