aw man that must be so hard when you have such a young baby to deal with as well
My 18 month old has been doing a similar thing for coming up 2 weeks now and it's exhausting. She had been a good sleeper for many months before this latest hurdle. She has always been really into her routines and I think she's not getting into the habit of waking up and not going back to sleep without cuddles. I haven't really figured out how to stop it yet, so don't have that much advice.
What I have found has helped the most is to keep an eye on the clock. Sometimes I find that I feel like I've left her crying for ages but it's only been 3 or 4 minutes.
I wonder if, like SusieC said, it's because of the new baby? Kids have funny ways of acting out their frustrations.
I really wish I had a better answer for you. Hang in there hun! And make sure you're getting as much rest as you can in the meantime til this passes- sound like you have a lot on your plate!