Confused!?!

Went to drs todays 37w6d...was chkd and dr said my cervix is very long and only 1cm dialated but thats moreso due to having previous children..now im confused at my cervical length u/s two wks ago revealed a cervix measuring so short the tech had to call my dr before being able to release me...ive had steady contractions the past 18hrs at 4mins apart which dr monitored and verified but says there intense but doing nadda for cervix...my question is how did my cervix go fron short to very long, and why arnt the contractions helping my cervix...also dr has me scheduled for iron infusion due to iron lvls at 9.3..my doctor said inducing me isnt an option, i dont understand why...