I hope you remember your date of last menstrual period correctly? mostly the disparities are due to incorrect recall of the LMP? If the baby is ahead of its developement by 3 weeks then you can undergo an oral GTT
Secondly the sensation that you are describing are contractions. They are painless in the third trimester i.e during the last three months of pregnancy
. If these contractions are painful and there is some associated bloody or water like discharge per vaginum then you must consult your obstetrician immediately.