Baby sleep issue, anyone been here before?

My little girl is just over 16 weeks old and has always slept pretty well at night, I feed so the regime has always been every 3 hours waking for feeding. The last three nights she has had her bath, been fed and gone down at about 9.30pm. She then wakes up between 11 and 12 and has been horrendous. She wont go back to sleep, she cries she grunts she squirms around in my arms and i am so exhausted I dont know what to do. She kept me up from 11 til 2.30 last night, not screaming so its not colic, she had that and grew out of it. I don't know whats wrong. I thought maybe she had silent reflux and am taking her to the doctors on Monday but i am so tired and so is she. But she wont sleep. aT 2.30 she went down and then woke at 4.30 and then at around 6ish she ended up coming into bed with me because she wouldnt settle for long in her cot and I really dont want her sleeping with me as its so uncomfortable and i dont get any rest. I really dont want to hear from people telling me that my sleep is not importan (i have heard that on here before) because if im exhausted then i dont function properly and in my opinion cant function as a mother and thats no good for her. Is it a growth spurt? Is it something that happens at 4 months? HELP!