Aww mama, that stiffness when you stand up sounds like it could be Braxton Hicks or just your ligaments stretching as your belly grows, super common! But since you're worried about it, I'd really suggest mentioning this to your OB at your next checkup, especially if it's painful or happens a lot, kasi peace of mind is important. A few of us mum friends experienced something similar and it turned out to be nothing, pero better safe than sorry talaga, so let your doctor check it out okay?