separation anxiety comes to mind.
I don't advise leaving her to cry - I did that to my eldest when he was 8 months (same scenario as yours) he cried then was very sick.
the next night I thought (why???) to try again - same thing, half an hour screaming then vomiting, even through his nose.
he's now 11, doesn't need too much sleep, has trouble falling asleep before 10pm.
we tried Jo Frost "bedtime routine" and Tracy Hogg Pickup/Putdown methods depending on age of child - they worked for us, but you need to do it properly.
I'd start on a Friday night, so whoever stays up can sleep the next day!
at 18 months I'd do this:
step 1.: do whatever you do to get her ready for bed, then put her in her cot, say good nigh/hug/kiss and step away from cot (you can try & walk out of the room,but chances are you won't reach the door before she cries).
step 2 .: she will scream so you go back immediately (crucial!), pick her up, no talking, just cuddle until she calms down
step 3.: as soon as she is quiet you put her back in the cot and step away
step 4.: repeat step 2 and 3 all night long like a fricking robot, until she falls asleep. if she does but wake s again in a few hours carry on again.
if she's just whimpering you can let her be, but stay there. use earplugs if must. count how many times you have to pickup/put down - the next night this number could dramatically reduce.
whatever happens do not get angry! take turns if it's too much or you just need a wee. you must both follow the exact same routine! no talking to her, seriously! if you talk to her she won't get the message
by staying with her she'll learn that you are there when you need her, but this is the only way to teach her to calm down and later on that she can fall asleep on her own.
you will probably have to do this for 3-4 nights, but you will see improvement soon!
read Tracy Hogg - the Baby whisperer