Hi mama! Sa 15 weeks, what you're feeling is most likely Braxton Hicks, super common sa second pregnancy kasi ang uterus mo na experienced na, kaya mas maaga mo nararamdaman, a few of us actually felt them much earlier sa second baby compared sa first! The tightening that comes and goes, especially when standing, and pauses when you change position sounds exactly like that, so I wouldn't worry too much, lalo na kung normal ang last check-up mo. But since you're feeling it almost every day at may slight pain sometimes, definitely mention it sa next OB visit mo para peace of mind lang, it's probably nothing, pero better to ask directly so hindi ka mag-worry the whole pregnancy, no? Take care mama!