Ay alam ko ang feeling na yan, sobrang confusing talaga sa unang pregnancy! At 18 weeks, ang matres mo (especially kung dalawa) ay talagang magiging irregular and lumpy pa, kaya normal lang na may hard spots na gumagalaw-galaw, most likely yan ay baby movements mixed with your uterus just growing and shifting pa lang, hindi pa actual contractions. But since may slight pain ka sometimes, best talaga na i-check mo with your OB sa next visit para ma-ease your mind, especially pag may dalang twins ka, doctors are usually mas careful with monitoring multiple pregnancies. One of my mum friends who had twins said the same thing around that time, and her OB assured her na totally normal, but she felt better knowing for sure, you know?