Always had chronic constipation since a baby. after bowel movement i sometimes feel if i push i could get more out but can not.had bit of clear mucus but it's gone now. odd time i get urgent feeling to poop but it's either gas or i can't.colon cancer?

No, colon cancer does not behave that way. A sudden constipated period can be a hint for colon cancer but not chronic constipation postnatally. The hormonal change during pregnancy does change your prenatal physiology and anatomy which contributes to constipation and your feeling of questionable rectal fullness after BM. You need to be active, eating high fiber foods and increase fluid intake.