Hi mama! Congrats sa 21 weeks, what an exciting time! Okay lang naman na ramdam mo na ang kicks every 1-2 hours, yan actually sign na healthy ang baby mo at active, lalo na parang laro lang para sa baby yan sa stage na yan. The pain you're describing, especially if it's just minutes at mawawala, could be ligament stretching or even Braxton Hicks contractions (practice contractions lang, totally normal), pero dahil may spotting ka rin dating at the sharp pain comes and goes sa sides mo, I think mas better na you see an actual OB soon rather than wait until 6-7 months, mamsh. A few of us had similar concerns at 21 weeks, at hindi talaga ideal mag-wait na long dahil mas peace of mind mo, plus they can rule out anything with a proper checkup at ultrasound.
I get na gusto ng MIL mo mag-wait, pero ikaw ang pregnant, your health and baby's health matter most, at 21 weeks is actually perfect time for a detailed anatomy scan. Go ahead at mag-book na sa OB mo mismo or another trusted one, kahit privately, para sure lang. Better safe than sorry, especially with the intermittent pain. Take care!