Oo, normal yan mama! Around that time kasi ang uterus mo ay lumalaki na talaga and tightening sensations (Braxton Hicks or just the ligaments stretching) ay common na, lalo na pag active ka or stressed. A few of my mum friends naman noticed the same thing around 18-20 weeks, so don't worry too much, pero best to mention it sa OB mo sa next check-up para peace of mind, especially kung may pain or spotting kasama nito.