It's just one of those things when you feel it, you'll know. Especially when you start feeling it more often. With my first pregnancy I first felt flutters right at 19 weeks. With my second pregnancy it was 20 weeks. Really does feel like gas bubbles though, or a butterfly trapped in your lower belly
You're not crazy. I felt flutters at about 17 weeks, but only a few times and I still wasn't sure that it wasn't just gas. To me it felt like little gentle taps inside my belly in one spot. At about 20-21 weeks I felt stronger movements that were much more obviously baby. My midwife said not to worry unless you still don't feel anything at 23 weeks