It could be a lymph node.
One time when I was taking a relaxing bath I moved a certain way and I caught what I thought was a glimpse of a small lump. I felt it and indeed there seemed to be something there (I am really thin in my stomach and pelvic area). I continue with my bath and saw it again when I moved a certain way. It was next to my pelvic bone.
So, I went in soon after to the gyno. He first did a visual exam and said he couldn't feel anything. Then he did the thing where they put that wand inside of you and he looked at me and said "There is nooo wayyyy you could have seen this!" Sure enough I had about a 5 cm growth on my uterus right where I told him I saw the lump!
Mine was a harmless lump and has not grown in several years. He checks it out each year at my well woman exam.
But, as always, we are all different and you should definately go see a doctor to be on the safe side.