What Causes Stomach Pain In An Infant?

Question: My baby is 6 weeks old and is otherwise healthy. I breastfeed her exclusively. She is a low weight baby at birth. I had difficulty gaining weight during pregnancy itself and she was born 2.89 kg. At 25 days, she was 3.37 kg. We have done the New born screening and thyroid test at her birth and all reports came positive. Our doctor told she is doing all good and normal.
Sometimes, her feedings are long and she gets hungry every 2 to 3 hours, but in the past two days, she has been feeding for 5 minutes and is crying again in 1 hour or 30 minutes. Also, she has been throwing up more than normal in these 2 days. I myself had some gas problems yesterday but it has become fine now. I called up emergency at her hospital and doctor told its normal for babies to do that some days. We gave her few drops of Neopeptin today morning and she has been less violent during feeds till now. Also, she passes motion thrice yesterday, usually she does it twice. Also, I found little mucous in her stool yesterday, which the doctor on call mentioned as one off incident and not to worry about.
Should I be worrying or taking her to Emergency?

Thanks for your reply. I have few more queries:

1. Her body seems to be little hotter than usual, but I checked temperature and it is 98.6 degree F.

2. I have noticed she passes some water with her stool which has a oily texture some days. She seems to be making an effort to push her stool. Her face becomes red. But stool is not solid. It is yellow mustardy. What is the reason for passing oil like liquid? Is it serious?

3. She seems to very voilent and angry while feeding. I know she is getting her milk. But she grunts and throws her hands and feet, scratches herself while feeding. What can be the reason?

4. She wakes up irritated or starts crying just before she urinates. Is it normal or is she having pain while urinating?

Please let me know.

Thank you so much for your reply. It is a relief.