Why is DD waking up so much?

[vvviola]

DD2 (almost 3) has never been a great sleeper - allergy issues at 6 months meant she woke every 45 minutes all night until she turned 1. Over the past 2 years things have very slowly got better. From about March this year we were getting the occasional time she was sleeping through the night. By the start of June she was sleeping through 4 nights a week, with only 1 wake up on the other nights.

Then she started coughing a lot at night waking herself up. after a month of it, GP reckoned she might have a sinus infection, so she was put on a 2 week course of antibiotics. Night time coughing improved, but she now seems to have lost all ability to self-settle - as well as waking up hysterical up to 5 times a night (screaming about totally random things)

I haven't had more than an hours sleep in a row in a week. She is warm enough (winter here), doesn't appear to be reacting to anything, has a full tummy and no coughing.

I can't just let her scream as she just gets more and more hysterical, but I equally can't go on like that (coincidentally I'm having sinus trouble too and feel rotten, which combined with lack of sleep is horrendous).

Any ideas/suggestions? (And no, DH can't help, only makes her more hysterical and I have to get up anyway. She's normally ok with him, but at the moment it's me or screaming )