Worried about newborn eating

This is kinda long, sorry.

Hey guys, my baby girl is 18 days old, she was very small when she was born because of an issue with my placenta, I was induced at 37 weeks and she weighed 4lbs 8.5oz. Very healthy, only problem is she is small. She lost weight down to 4lbs 1oz, but last week at the doctor she was back up to 4lbs 6oz. I have gradually been increasing the amount she eats per feeding, and now she usually eats 1.5 ounces every time for a total of about 10 ounces a day (over twice the amount she was eating her first week home). She will not latch so I have been pumping and fortifing my milk with formula for extra calories as recommended by her doctor. For the past two days she has been going through a growth spurt, and will drink up to 2 ounces each feeding. I am not producing enough milk for her now that she is eating more, and today I had to give her formula for a few of her feedings. Usually she eats every two hours. Four hours ago, she drank 2 ounces of fortified breastmilk. I changed her diaper and there was no poop, which is a first. Two hours later, I tried to wake her up to eat, and it took 45 minutes for her to eat half an ounce of formula. She acted like she didn't want it so I finally gave up and decided to let her sleep for a little longer to see if she would get hungry then. An hour later I tried to feed her again, and she did the same thing. Basically refused to wake up and eat. I stripped her down to her diaper, changed it, and aggravated her until she woke up a bit, but this time she didn't even drink half an ounce. A few minutes after eating, she spit up a bunch. Her stomach is tight and still no poop, I let her lie back down and figured maybe that 2 ounces earlier was too much for her. I gave her a couple gas drops and now she keeps grunting and farting in her sleep. Do you think it is just gas, or she just got too full earlier? I'm worried. I don't want to let her go too long without eating because I don't want her to lose any weight. But when I try she shuts her mouth tight and will spit up anything I do manage to get in her mouth. I usually never have this problem, she's usually hungry every two hours like clockwork. She was fine earlier today too. Anyone know what might be up? The pediatrician office is closed right now, but I plan on calling first thing in the morning if she keeps on like this. :(