Mama, please have your baby checked by your pediatrician as soon as you can, stiffness combined with that kind of crying during startle needs proper assessment kasi hindi kami dapat mag-assume dito. One of my mum friends' baby had similar concerns and the doctor was really helpful in figuring out kung ano talaga ang happening, so don't worry, okay? It's better to get it checked na para mapakali ka and ma-rule out anything serious. Keep monitoring din how often it happens, para pwede mo i-share sa doctor. You're doing great, mama!