I felt it I know I did!

I am 11 weeks 4 days and I am very thin build, this is my second baby. While lying flat in bed last night I felt a hard ball sick out of my lower abdomen, my partner felt it and it then moved to the middle of my abdomen and stuck out again, this continued for about 5 mins. If it's not the baby then what else would be moving, if you feel it will your hand it feels really firm where it sticks out. Anyway I googled this and it says that some women on a second child who are very slim are more likely to feel this. Anyone have any opinions on this or what it could be?