Mommy, yan ang best na sundin ang pelvic ultrasound date mo kasi mas accurate siya, go with September 2 as your EDD! A few of us here actually had different dates from LMP vs. ultrasound, at ang doctor lagi ang nag-guide based sa scan.
That said, with those pains you're describing, ang tiyan paninigas, back pain lasting minutes, please mention these to your OB ASAP or go to the hospital if they get worse or more frequent, okay? Hindi ito normal na ignore lang, lalo na ngayon. Better safe than sorry, especially on your second baby. Keep us posted, sis!