40 weeks pregnant and constipated?

I am 40 w +2 d pregnant and am 4 cm dilated and baby is EXTREMELY low. I have had 3 membrane sweeps done and they don't seem to be working. My entire pregnancy haven't had any constipation issues. This past week i was having frequent bm ( at least 2 a day sometimes 3) suddenly today I have had NOTHING but i constantly feel as though i have to go. my baby's head is SO low that if i do TRY and go it feels like i'm pushing out baby (yikes!) and it is painful to try. My doc has been monitoring me and baby for about 4 days now and baby is perfectly healthy with a great heartbeat and movement. What would cause my body to suddenly be constipated and what can i do?