This is baby #4 for us, and I THINK I've felt movements since about week 14, though it would just be one little movement every few days and then nothing. Now, at 17 weeks, it's usually at least once a day and sometimes more one or two movements at a time (and only when I'm sitting/lying still). To me at this stage, it feels sort of like a quick flutter, sot of like a bubble bumping into another bubble (that made more sense in my head than writing it out!).
If you have a good sense of where your uterus is, it should help you focus in on where you should be sensing movement -- at 17 weeks, the top of your uterus is probably a couple of finger breadths below your belly button, and you should be able to feel the sides, especially when you're lying down. Movement above the uterus might be gastro-intestinal; movement on the sides might be muscles/ligament stretching.
In a few weeks, you should be able to know for sure whether what you're feeling now is baby movements -- I remember with my first looking back and realizing THAT'S what that was!