4 Month Old Sleeping Problems

My 4 month old baby girl is STILL up every 1.5-2 hours at night to eat and I'm so exhausted. She was this way at birth, but that quickly changed and she started sleeping 5-6 hours at a time. Now this past week she's up at least every 2 hours. She is formula fed during the day at daycare because I can't pump enough, but all evening at home she is breastfed. I've tried giving her formula at night to tide her over longer but it hasn't worked. I'm not sleeping and neither is she! She gets to the point during the day where she's so exhausted that she'll just pass out. I'm getting a little concerned about this. Any suggestions? :/